Aim for around 100 new Pokemon total, including evos-prevos of previous Pokemon.
5 or 6 legendaries total, with 1-2 of those being event only.
Don't go by hard numbers, just use those as an approximate target.
I agree with
>>16052304 that the main problem is the bloat of legendaries, especially when they would be more thematically appropriate for future regions. Like, Deoxys would've fit better in Gen IV, the Swords of Justice would've fit better in Gen VI, and so on.