* ATTN - POSSIBLY GLITCHED ACHIEVMENT - READ BELOW FOR INFORMATION TO HELP POSSIBLY UNLOCK IT. *
This cheevo may not unlock when it is supposed to. I would recommend making a backup save before making the purchase of the 50 unit roster spots upgrade item from the armory. I am unsure if you can wait on the purchasing of the 6 squad spot upgrade as well. If you can wait on that one, I would try to make the purchase of both upgrades at the same time if possible. I'm thinking the upgrade availability is based on how far along you are in the storyline so if you wait on doing upgrades as long as possible, you may have a better chance of this unlocking.
You will need to have the following unlocked and purchased before the achievement pops :
5 squad spots - 1000 food.
6 squad spots - 2000 food.
25 unit roster - 1000 food.
50 unit roster - 2000 food.
In order to unlock the 50 unit roster you will need to progress along in the story about 1/2 the way to unlock the 50 unit roster spot . I was around 45-50 missions completed before the last item was shown to me for unlocking but I was also doing every mission possible since I was unsure what I needed to have done before it unlocked.
Thanks eohjay for the exact food requirements for each level.