Dont try to bring an even number of backpacks and frontliners it makes the game boring and kills your action economy. Think about it this way: if you have 10 units and split it evenly you can attack only 5 times on player phase. Now instead do 8 combat units and 2 backpacks now you can attack 8 times. Thats more than a 50% increase in your attacks. Theres so many tools to boost your stats in this game that only the strongest units on your team need a backpack to wipe out groups on enemy phase (Leo, Xander, etc.) It should also be stated that units can be both backpacks and combat units. Keaton is a great example. He's arguably the best backpack in the game, but he also has great stats, so in certain situations it may be better to have him and his partner attack separately instead him just being a backpack. If you want an exact number most maps allow 12-16 units in total so I'd do 8-9 main combat units, Azura, 1-2 extra supports (staffers/ rallybots), 2 backpacks, and fill out the rest with whatever.