I started a thread about this a little while back-
I found that around 25 is common. Spoke to abot 15 friends/friends of friends who teach prep/reception and year one and the numbers were 12-30 in a class. most sat at about 24-26.
The school that had 12 has a max class of 15 and charges nearly 10k per year.....
I'm in SA for reference so thats where my numbers have come from