I think counting is a bad idea. It's tempting to keep a scorecard isn't it??!! league tables! Stars!
But it helps maybe to remember that it's not an equal playing field after all - pardon the pun.
My DD (also an only and 5yo)is often asked for playdates, as she's popular and behaves well, she's imaginative, and creative and her friends like her. She's very good at playing, and doesn't need my input mostly.
However, we live in a two bedroom flat, with no garden, and an office. We are just not set up for suburban exchanges - we are urban and we don't keep a car, which limits our range a bit - public transport and then sometimes quite a long walk - then having to hang around? or commute?
We have had a few girls over - singly - and I was really gobsmacked at how what I'd consider, badly behaved they were - and how very messy - pulling things down from shelves on top of themselves, picking up ornaments (which are not toys, you know!! ranty ranty ), always coming up to me to get me to organise them - like they've never been able to play independently... they're not really house trained IYSWIM
So I NEVER invite kids round to ours anymore. It's not worth the hassle TBH.
When it's time to meet up and play with DDs friends (and I'm quite aware that I'm in charge of her social life ATM), we meet in a neutral, safe place like the local playground, or a museum, and I keep my distance, so they get to socialise with each other.
Obviously I'm around, but I see playdates as her time with her friends. Sometimes the other parent/s come also, and we make small talk from the sidelines.
I do understand where you're coming from though - I find parents of kids with sibs a teeny bit insensitive when it comes to the social interactions. Perhaps we 'count' playdates more as we are parents of onlies?
I've learnt to have a bit of a thick skin - people cancel at the drop of a hat also... what can you do, eh? It's not like there was a contract! I know where you are coming from, but people are mostly just doing their best.
Some of my DD's friend's parents are splitting up recently and that little friend has just been off the playdate 'scene' for a while.
I mean, in this instance, I get on better with my DD's friend's dad, but I obviously just can't ring him to arrange a playdate... so my DD's relationship with her little friend is on the back burner for a while. There are many factors involved as to why the invites aren't coming in thick and fast, you know.
Keeping score is a recipe for wrinkles I think