There are set percentages of your total allowed for tech and military, and there minimum gross numbers to be able to sell (5k for military, 10 points for tech). The percentages vary by server and government.
In tourney you need 20,000 of a military unit in order to be able to sell 5,000. Five thousand is the minimum number of units you can sell and you get to sell 25% of those you have.
This is important for start ups, that is the way to play the first 100 - 120 turns. One of the considerations is getting to 20,000 units as soon as possible after emerging from "protection".
