When my girls were that age there was an unofficial £10 limit which every parent seemed to stick to. The first couple of years of primary will have some hosting whole class parties, but after that the numbers drop to single figures rapidly once the kids make friends.
Same here too, when my little uns' were at Reception Age and early infants the amount of parties was relentless, felt almost every other weekend, but as above it'll soon dwindle to smaller affairs with your child's group of friends, more recent get together's now seem to consist of a smaller group being taken to the cinema, bowling or other similar activity at birthday times.