I'm a Reception teacher. I would advise 5 sets if you can afford it, then you won't be rushing around too much trying to get everything washed and dried in time. He will definitely need clean clothes every day, as he'll be using paint, sand, playdough, mud, whiteboard pens etc, plus no doubt getting his lunch down him too. Spare pants, trousers and socks in his bookbag are also advisable if you anticipate toilet accidents.
Ask about PE kit in advance. If they go in their kit one or two days a week you'll need less school trousers and some joggers and trainers instead (many schools still use the school jumpers for PE).
My advice would be to buy no more than 3 logo jumpers. They're so expensive and most schools accept plain supermarket brands so, even if you'd prefer a logo, a couple of plain ones will be more than adequate as back up.
Name absolutely everything, even things like shoes if you can. So many will have exactly the same pair! Coat, hats, bookbag, trousers, jumper etc. You name it, name it!!! The iron on stickers you can order online are great for this but a sharpie will also do fine.