The more you add, the more differences in interpretations - you get plenty of breaks in a 3 if you do it properlyI personally think 4 field umpires would be best suited, you could have one in each inside-50 mark and 2 in the middle with one on each wing/half middle space to help umpires have less work to do.
Anymore field umpires than that though and it would probably affect the state of game, in a negative sort of way.