In general, a character can initiate only one psionic power per round. There are two key exceptions:
Once a power is initiated, a character can maintain that power for as long as he can continue paying the maintenance cost (this assumes that the power has a maintenance cost; if no such cost is mentioned in the description, the power cannot be maintained beyond its initial duration).
Psionic defense modes (mind blank, thought shield, mental barrier, intellect fortress, and tower of iron will) do not count against the one-power-per-round limit. A character can always use a defense mode and initiate one other psionic power in the same round.