Staff of Ra

Does Ra reroll all his automisses once, or does he keep going until he has rolled no automisses?

Once

You pick up all the auto misses and re-roll them in one roll. 

Welp, been playing THIS one wrong...it does say "all," though...

You have to think of it as a triggered ability. It activates when the "When" condition is met. In this case, "When you make an attack." Basically, you roll, this triggers, you take all the automisses and reroll them. The ability doesn't trigger again on the reroll, so you're stuck with it.