Our school has a cupboard full of spare uniform for changing them after toilet accidents, and prefers to use that than have the children bring their own spare set in. If they get muddy/painty etc then they just come home like that (and usually with it on back to front if they had PE, too).
I have 2 school jumpers and a cheapy plain one for when I don't do the washing. And 4 pairs of trousers, 3 would have been enough but they came in 2-packs.
How often are you going to do washing? And do you care if they go in with slightly muddy trousers from yesterday? My elder two are 6 and 9, and as it is their responsibility to check their clothes are clean when they put them on, they go in messy quite often .
i'm in the same situation. i got 2 2nd hand nursery jumpers and trousers for messy spares, and have ordered one new jumper. the trousers and polo shirts i can get from the supermarket so will get a couple of new ones and see how we go. hopefully that should be enough! i'm going to send in spare pants and trousers, we've never really needed more when out and about usually.
i'm totally not ready for this! (and neither is he )
I have a Mr MuckyPuppy so I just went for 5 complete sets of polo/jumper and jogging bottoms, thankfully mostly second hand. I'm a F/T WOHM so being able to do all the laundry at the weekend is useful and with luck (as he's at a different non-uniformed nursery in the afternoons - don't ask) I can save some laundry by changing his tops at lunchtime.
The nursery school require one set to be in the regulation shoebag for spares at all times, so I just have the oldest 2nd hand set in there (they changed the school logo a while ago, so some of the 2nd hand stuff is obviously several years old and I didn't realise when I went to the sale! )