Generally it is only the head coach permitted on the field. Of course, in the younger age groups, where learning is stressed over competition, that depends on the league. But from what I have seen, the number is limited to one.
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Follow up by: Dan
A reader (thank you Richard Beals) that there is no limit to how many coaches are permitted on the field. Under further inspection in reviewing Rule 5 - The Game, Section 7.A, the offensive conference refers to "Manager and and/or other team representative(s)". For Defensive Conferences, Section 7.B, it refers to "manager or other team representative". So it appears that only one for defense (mound meetings) but multiple for offensive.
As I always state, this may vary by league, so check with your league officials.