For Shield ability, does Riot Trooper gain 1 power token per figure, or just 1 total?
Each figure would gain a block power token.
In addition to what the original rulebook says on page 2, page 4 of Heart of the Empire's rulebook, says this about power tokens:


  Various cards and abilities in Heart of the Empire allow figures to gain power tokens.


So power tokens are bounded to a figure and not the player who controls it.
In this case, if you control two Riot Troopers who depleted their block power tokens, both of them would gain their own token. If controlling two Riot Troopers and just one of them lost all of its block power tokens, he would win a new one.