The delayed triggered ability created by Jace's first ability can trigger more than once in a turn if creatures you control deal combat damage at different times in a turn (most likely because one or more has first strike) or if creatures you control deal combat damage to more than one player at once.

If a token created by Jace's second ability becomes the target of a spell, its ability triggers and goes on the stack on top of that spell. The ability will resolve first (causing the token to be sacrificed). Unless the spell has another target, it will then be countered when it tries to resolve for having no legal targets.

The tokens from Jace's last ability copy exactly what is printed on Jace and nothing else. They don't copy how many counters are on him or Auras attached to him, or any non-copy effects that have changed his color or types, and so on. They'll each enter the battlefield with three loyalty counters. You can activate one loyalty ability of each of the tokens this turn.

The tokens created by Jace's last ability don't have the legendary supertype. If another object becomes a copy of the token, that copy also won't be legendary.

You can control one legendary Jace, Cunning Castaway in addition to any number of nonlegendary copies of Jace, Cunning Castaway.