The maximum amount of allies you can use in attack or defence is your current level x 5
Example, if you are level 20, you can use a maximum of 100 allies (level(20) x 5(allies))

The maximum number of units you can use for attack and/or defence is your current level x 5 allies x 6 units
Example, if you are level 20, you can use a maximum of 600 units (level(20) x 5(allies) x 6(units))

If your alliance is lower than the maximum then each ally will utilise a maximum of 6 units each
Example, if you are level 20 and you only have 25 allies you can use a maximum of 150 units (25(allies) x 6(units))