So when running with a tie swarm and you're keeping them in tight formation (for this example we will say 5 tie fighters), when attacking how many rerolls do you get? One all together or do you get the option of one from each tie fighter in the swarm. So possibly 4 rerolls on one attack or not?

It is a "While" ability. "While" abilities are only triggered one time.

So, a Swarm squadron like a TIE Fighter would get 1 reroll because is states "you may reroll 1 die" when the target is engaged by a friendly squadron. This can be a TIE Bomber.

Just to make sure I'm clear on this. I would not be able to reroll multiple dice on a single attack for each tie fighter in the swarm, just one die overall can be rerolled.

Basically 1 die per attack

Basically 1 die per attack