To think it can be hard to explain how many siblings I have?

I'm fine with "it's complicated"
It prepares people for when the conversation stops abiding by the conventional rules of logic

Otherwise count the relationships that mean something to you. I'm guessing that you're not that invested in the steps of a parent you met in adulthood and they add little to small talk.

Some good suggestions from PPs for how to answer this.

But much sympathy for the thought that has to go into navigating these small talk questions, where anything other than a very straightforward answer risks making you seem attention-seeking or a bit of a downer.

Family based small talk requires a lot of conversational strategising for anyone who has lost a child / grown up in care / has any other difficult or unusual set of circumstances, so as not to make other people feel uncomfortable.

I don't ever launch into this BTW when I know someone is making chit chat. I normally say something like 'I was raised by my grandparents, they had 11 kids'

I think this is an absolutely perfect answer and you should keep saying this. It reflects your actual experience, makes it clear that the situation is complex (and so perhaps not something for idle small talk) and is entirely honest. If you wanted to be even clearer you could say 'I'm my mum's only child but I was raised by my grandparents, they had 11 kids'.

I think you're overthinking this. "I'm an only child but grew up with loads of cousins/aunts/step siblings at various times so sometimes it feels liek I have loads! What about you?"

I would say 'raised as an only child, my mum only had me, but there are step/half-siblings from both sides of my family.

Depends on why you're being asked, if it was in an ice breaker style approach I'd probably say "two half siblings, though im not close to them. Massive family though inc step siblings so I felt like one of the Waltons".

If you were answering factually, two half siblings/one of three.

What @BleedingLikeABastard said — it depends who’s asking. If it’s a small talk question at a work icebreaker or a slight acquaintance, just say ‘It’s complicated! And you?’ If it’s someone you are happy to confide in, and you’re not pressed for time, say what you’ve said here, or a version of it.

Did your aunts and uncles feel like siblings? Were you treated as the youngest sister?

If yes to both, say you were taken in by your grandparents and then grew up in a big family where you were the youngest of 12.

When people ask about families, they don't really want the biological details. They want to know how to continue the conversation, and what sort of childhood recollections of their own they should swap with you to bond over.

There’s your answer -possibly- you’re one of twelve, because that’s what it feels like to you 😊 It doesn’t really matter if the people you grew up were not strictly speaking your siblings, if you felt you had those types of bonds.

Say "how long have you got", and whip out a family tree 😀. Personally I'd find it fascinating 😊

If you are the youngest of 12 you have 11 siblings. Though it's confusing as you say you are your mums only child.

Sorry, didn’t see the ‘if they weren’t’ bit. That’s a question made impossible by the fact that the people who were your guardians were also your grandparents.
If you’d been adopted then they’d be your adoptive parents (and yes, you’d hopefully just think of them as Mum and Dad and your brothers and sisters). But being blood relatives blows that out of the water rather…

An absolute gem of a relation of oofs (genuinely) has had several step dads and mums and lots of bio and half and step siblings.
When asked how many siblings he has he just replies 'loads'!! When pressed he just says 'it gets complicated because I've got steps and half's but they are all great.'
That works for him

I'd go with something like "I grew up in a large family, there were 12 of us"
It doesn't specify siblings but identifies them as family to you.

@howmanysiblings I live hundreds of miles away from where most of my siblings are or it would have been a consideration.

I was brought up with 2 siblings, then found my dad some years later, then discovered through various stories of his that he has somewhere in the region of 20 kids, he never answered exactly how many, I'm not even sure he knew.

I had double figures of aunties and uncles and hundreds of cousins too.

Weirdly 3 of the siblings have given our oldest 2 dc the same names as I found out.

My family is a mess.

'I was raised by my grandparents where I was the youngest of 12'.

Then I'd say that's your answer then. Count the people you see as siblings as siblings and just give that number. Although, given that there's so many of them you could probably just get away with "A metric fuck-ton", and avoid having to give an exact number.

No, they’d still be your aunts and uncles, just as your grandparents (while doing an incredible thing in becoming you legal guardians) would still be your grandparents. If my partner and I die before our daughter reached independence then my BIL and his wife would take her. They wouldn’t become her parents. She might look back and say that they were wonderful and treated her like a daughter and she grew to think of them as substitute parents, but they would still be her aunt and uncle.

I'd say "My family structure was a bit complicated, but I grew up the youngest of a big bunch". That describes your actual experience of sibling-like relationships, without lying. Then you can go into more detail if it fits the conversation.

I would say I am my mother 's only child but I was raised by my grandparents where I was the youngest of 12.

I think people are being very literal about this. If it's part of small talk, you don't have to stick to the most literal, accurate, biological truth. Even if there was only one way to understand 'sibling' (I'd argue that all family relationships are social as well as biological), it would still make sense to answer based on what you actually want to talk about. So OP you should say the one that feels most like you, and where the follow-up questions actually lead to a nice chat.

I'd say 'i'm an only child but have various step and half siblings'

Them: “How many siblings do you have?”

You: “Two”

Easy and true

I’d just go with “I’m the youngest of 12” or “none”

Keep it simple, nobody wants complicated details of your family tree.

EDIT: I changed my answer after I realized I misread the OP