My dd is 6 but things haven't changed - she doesn't really have any normally! dh and I both work though (me 4 days a week) so it would make it tricky. I don't think it matters so much at that age, let them settle in and see how things go.
Usually 1-2. 3 is too many I find. We don't do any other activities. But at the beginning it can take a while to warm up and you have to be brave and do the asking if you want to do some. I didn't always wait for the kids to arrange them.
I teach 5 year olds and they seem to at each other's houses all the time. My kids are older now (youngest is 7) but they have playdates about twice a week then have other activities such as sports on top. Bloody exhausting for me
Last year lots but I think most of DS's year group are shattered after a full day in reception. Its probabyl a bit soon tbh. All DS wants to do is eat, drink & crash in front of Cbeebs for 20 mins at the moment. Think as th eterm progresses we'll get together maybe once a week with someone.