We have 4 and baby number 5 is due in the next 2 weeks.
We have :
DS1 - 16
DD1 - 14
DD2 - 8
DS2 - 4
DS1 is just going through choosing what he does next but will most likely stay for 6th form and then off to uni and is expected to get 5-7's in his GCSE's (so C+'s to A-'s).
DD2 is currently expected to be getting 6-8's but obviously we have 2 years left for her.
I don't think that a large family always equates to poor performance and therefore lower paid jobs however that said 21 is extreme!
DD3 was a very unexpected addition and I think if they had all been under the age of say 10 we wouldn't have continued as we just wouldn't have the time or energy for them all.
The elder 2 occasionally look after the younger 2 but it is only occasionally and for no longer than an hour and TBH we see a lot more families of say 3 who leave their younger ones with the teen/s than we do. The elder 2 also don't do anything around the house other than basic chores of washing up and the other dries up and puts away and tidying their own bedrooms.
We have found, that like most teens, the elder 2 only want us when they are hungry or want money lol but all 4 do get 1-2-1 time with us. The younger 2 every day for reading etc and the elder 2 as and when they want it.
We have a 4 bed house so until July the younger 3 will share one room (for around 3 months as she will be in with us until shes around 3 months old) as DS1 is doing his GCSE's this time and will need his own space but then DS2 will go in with him but we are planning on building a 'summer house' for the elder 2 so that they have some private space but I have no issue with 2 kids sharing a room. I had to share a room with my brother who was 6 years younger than me until I left home which wasn't ideal (especially when I started puberty) however I think we'd have been ok if he'd been a girl.
For us it works however I can't imagine how it's possible with 21.
From a lot of posts on here we, also, are excessive and selfish and can't give our children the time & effort they deserve & I think that it unfair and rather judgemental. Sure our life is busy and demanding & it comes down to having a relatively strict routine and preplanning but we do more with our children than quite a lot of people do with 2.