As for the order, the rules don't say one way or the other. Most people which I have played with do it by weapon type within a unit firing first though. EG: 2 SM squads with plasma weapons, roll to hit, wound and save (if applicable) with unit A's plasma, then unit A's bolters, then the same for unit B.
As for a limit on the number of saves/turn. There isn't one, a save is taken for every single successful wound unless the weapon denies the use of saves.
Keep in mind that if the unit targeted has mixed armour you need to roll saves in batches.
As for a limit on the number of saves/turn. There isn't one, a save is taken for every single successful wound unless the weapon denies the use of saves.
Keep in mind that if the unit targeted has mixed armour you need to roll saves in batches.