Welcome to Librarium Online!
I was wondering if the re-rolls given to you by fleet commanders, mark of tzeentch etc are re-rolls a turn, or if those re-rolls were one use abilities.
One use ability. You use it, it's gone.
Nights Justice Space Marines
Eldar Eth Kariel Craftworld
Spear of Kurnous - High Elf Expeditionary Army
As has been said a BFG re-roll is a one-shot thing, so you only get to use it once per game. Thats what makes buying re-rolls for your commander so useful, as they can really help in a tight spot.
The one exception to this rule that I can think of is Abbadon. If he is taken as a fleet commander then he grants one re-roll per turn; most useful indeed. Only usable by Chaos though; I've heard Imperial Navy personell often dislike being commanded by the Warmaster of Chaos.