Should I gather a cloud of units?

A dense swarm of units is one of my favourite configurations. That said, by taking time to gather the swarm you are surrendering the initiative to the other team. They will expand and get more map.

Therefore you either need to make sure that the units that you gather can decisively counter what the enemy is using. That means that you will need to pay careful attention to what units your opposing players are using and you will have to cloak the swarm until it gets into action. There's nothing worse than having a swarm and Jummy or Sab comes to visit with likho.

Or you have to gather a cloud while still maintaining the offense.

It's possible to do that but then you either need units that hit hard and self heal like shields or you need high toughness units that you can retreat and heal like Minotaurs.

You should use your units if and when they are likely to win.

If you can win with one glaive, win with one glaive. Even a single glaive will win vs a Mex. So sending two is wasteful.

If you cannot, but you can win with twenty, then wait until you have twenty. A single glaive will not win vs 5 Scalpel, so sending one is wasteful. But 20 glaive will. And you won't get 20 if you keep sending every first one to die to do no damage.

An important thinking-tool for this is lanchester's square law. Even if it is not perfectly square in ZK, sometimes the outcome flips depending on how many units you have, and sometimes it flips critically - from marginal defeat to decisive victory.

Also re Anir: keeping units alive is a good thing, but if sacrificing all of them nets you victory, you should. The issue is, of course, knowing whether it will bring you victory or not. I tend to bias towards yoloswag, and there have been plenty of games i've lost over a factory surviving on 0.1 hp.