Very difficult to answer!
Obviously depends upon where you live/ your other expenses and what type of lifestyle you want to maintain.
I can only really speak for us, but if that is helpful...
DH works f/t and earns approx £85K + bonus (last year 30%, this year 0%!), I work p/t and earn about £10K (doing a bit more soon, so £20K). We run one small car between us and have a small mortgage (£90K).
We could easily afford to pay pre-prep/prep fees for one child and maybe two, with a bit more effort. Our DC goes to state school though, as we think it is better than the local private school.
Our surplus cash goes on overpaying the mortgage. It should be paid off well before our DC goes to secondary school, so private school (which would be more expensive then) would be a comfortable option for one, or even two dc. (DC will probably go to the state school though).
TBH I would feel a bit stressed sending two children to private school on our income right from pre-prep, but I am an over cautious type!
I know a few SAHM's (casually not really friends) who have 2 or 3 children at private school and I always think (never say!) their DH's must be earning at least £150K to even begin to afford that.