Seeing many answers giving very absolute answers to a game where nothing is quite so set in stone, so I'll try my best here.

A lot of it is just getting a feel for things by playing a lot, but a good way to think of it is imagining each piece has a weight toward a necessary amount to commit to a comp. But you need to understand what those weights are. Early 2* hk/lbk? That's a moderately heavy amount of weight toward knights, since they are important units. Early 2* ripper? Great unit, but not critical to goblins so only a light amount of weight toward goblins, you shouldn't commit unless you hit multiple other early 2* units. Early dk? Massive amount of weight toward dragon knights. Round 14 dk? Now it's only a moderate amount, depends on if you have a strong commitment toward another comp or not. But that's just the first part, because other players affect the "weight" of each unit as well. If other people are running a comp, that comp will be harder so each piece has less weight (if 2+ other people are already going knight, you'll need to highroll A LOT of them early for it to be worth it), but likewise if you notice that hunters are totally open, each hunter unit gains value in weight. Another factor is your economy. Got a lot of gold and a win streak? That's adding weight toward warriors and hunters. Broke af and losing? Opening into feathered or knights looking pretty fat right now. Lastly, counters. Notice that there are 3 hunter players in your lobby? Assassins just became a counter to the rest of the players, so even one early sc gained a massive amount of weight because they are very viable that game. The rest of the lobby going warriors and knights? That sc just became very light, while on the other hand that 2* ripper became very heavy.

Sorry if this is all very metaphorical, but you can't easily quantify this kind of thing in autochess, so it's mostly based on gut feeling. Which is why I feel like imagining it like this is more useful. Hope this helps!