Trip can be used in two ways, a standard action attack type or a full round action attack type.
For instance: a level 6 soldier has BAB of +6/+1
Standard Action Attack limits him to 1 use of trip.
It can be used only once in an attack or charge.
If he uses a full round action he can use it more than once, so in the case of this level 6 soldier he could make a trip attempt at +6(+mods) followed by a normal attack at +1(+mods). Or he could make two trip attempts.
Trip can also be used as an attack of opportunity with normal restrictions. (Character normally gets 1 AoO per round, combat reflexes would grant this char multiple uses of trip from AoO)
