Reinforcements

There is no reinforcement mechanic in SotS2, you fight with your entire fleet so what you bring is what you get.

In SotS1 though it's a bit different, your command ship act as a beacon to your ships so you can invade a planet with stacks of doom and when a ship is destroyed a new one will automatically warp in on your command ship.

Warder

In SotS2 your entire fleet attack at once, including, your supply ships and what not.

You can however, send several fleets to attack the same system and it will trigger several combat rounds in the same between turns time, so for example, if you send two fleets you will be able to attack with one and than with the other.

The other day, I attacked one of my brothers heavily industrialized system (4 colonized planets, 2 asteroid belts and 3 gass giants!!!) I used two fleets, a cruiser fleet with the fastest ships I could build and a DN fleet with four BR carriers and a command ship, I used the cruisers to grab as much sectors as I could on the firs round while keeping my DN away from his fleets and planets, next combat round I deployed one carrier next to each cluster of mining stations and systematically destroyed all of them while my cruiser fleet remained hidden.

I lost a DN and several cruisers for that but in exchange he lost the ability to build his 2 DNs in each turn.

Warder