As far as I'm concerned 10 is extremely generous and still includes several vanity candidates who know perfectly well they stand no chance of winning a single state primary or caucus, never mind being nominated.
A "debate" with 10 candidates winds up giving each a few minutes of air time, just enough for a few sound bites without substance and a lot of time wasted on "I agree with what so and so said..." type of discussion.
I agree 10 is generous but I'm only talking about the first debate. After that it should be reduced by one or two each debate going forward. If there should be a candidate who cracks the polls from outside matching the criteria they should be included.