In general I'm quite defensive of larger (4+) families in general and I don't like the assumption that more kids must = worse childhood.
I am good friends with a lady who has 5 kids and puts a huge amount of effort into one on one time with them, helping them to enjoy hobbies they're interested in, listening to them, helping with their education. She is totally devoted and her kids are 100% the focus of her life, they are all very happy. I look at her and I look at mums with only 1 or 2 kids, where the parents are just not engaging with their kids at all, ignoring them and spending hours and hours on their phones, packing their weekly schedule so full with things (I'm not just talking about work) that there isn't actually any time left to spend with their child, and just generally resentful of having to "make room" in their lives for the needs of their child(ren).
In general I think that it's the quality time and effort that you put into raising your kids that matters. There are only children out there who aren't getting their emotional needs met by parents who don't prioritise them in any way, and larger families who are doing a great job.
Saying all of that there are only so many hours in the day. Realistically it must be impossible to give 21 children the support and 1 on 1 time that they need. Surely?
I have 3 kids and it takes a lot of effort to make sure they're all getting what they need from me emotionally, and that the ins and outs of their lives is important.