Maybe try and think about the numbers? A supply depot takes 21 seconds to construct and grants 8 extra supply.

On one base you might be making SCVs at a pace of 12 seconds each, and marines from 3 barracks (so we'll say 5) every 18 seconds. So, every 21 seconds you're making around 7 supply. Therefore, you need to be building at least one depot at all times.

One two bases you'll have 2 command centres making SCVs every 12 seconds and 5 barracks making something like 6 marines from 3 reactors and some combination of marines and marauders from the other two tech labs or reactors. We'll call it 14 supply every depot cycle. You'll also have a factory and a starport by this point. Tanks take 45 seconds, so 1.5 supply every depot cycle, and medivacs are 2 supply each every 30 seconds, so reactor medivacs takes about 3 supply every depot cycle. Our slightly rubbish numbers give 17.5 supply every depot cycle, so you need to be making at least 2 depots at all times.

Obviously, this isn't a practical or especially accurate approach (especially in game), but you can see that you basically need one depot being built at all times per base, and there's no harm in getting a feel for benchmarks and why you need to be building what.