Armada

Fleet Supply is essentially a number of points that limits how much ship power you can field at once. Depending on the size of your empire there will be a limit (e.g. max 2000), and each ship that is alive at the time will cost some points (e.g. 50 for a capital ship).

For this achievement, you must use all the way up to the limit. In one sense this is easy (2000/50 = 40 capitals) - but if you end up with some ships with an odd number of points then you can get stuck at 1999/2000 or similar and end up needing to scuttle/suicide some ships before then strategically choosing what to build to end up using 2000 exactly.

Easy enough to do - but in many matches it will need a little deliberate effort to get it unlocked.