there are a few possibilities:
1 gown - throughout dinner. usually a designer white gown, so only have one. ( plus point - more time to spend with the guests)
2 gowns - the most common, one white one and 2nd march in with an evening gown.
3 gowns - 3rd one usually is a send off gown. some couples do performances, such as dances, so that's an extra in between the 1st and 2nd evening gown (usually dances have short gowns, or more flowy for easy movements)..