The number of cards in your hand is checked only as the triggered ability of Ral’s Staticaster resolves. If that number changes later in the turn, its power is unaffected.

Whether you control a Ral planeswalker is checked as the ability triggers and as it resolves. After it has resolved while you controlled a Ral planeswalker, Ral’s Staticaster keeps the bonus for the rest of the turn even if you lose your planeswalker.