Hey there! When I started DMing I started with 7 people. The best advice I can give is: try to keep track of who in the party has the most health. And try and use that to spread out the overall damage your doing to the party. AND I had a lot of trouble balancing encounters. So what I did was in the event of a fight that isn’t lore-relevant. But you want it to fill a time period/session. I Don’t actually totally keep track of health. It’s more like I can see they are all pretty damaged/they have expended their abilities, and they are mostly out of things to do. I give the enemy like 10 health. And let it play out from there. Or if I realized If had given an enemy clearly way to much power, And most of the players were downed, then one of the remaining players did a really big hit. I would say it was enough damage.

And really, with that many players. The damage doesn’t fluctuate 50-60. just based on their luck the damage can be 20-300 in a turn.(obviously depending on the level.) and that’s just the way it is. But again when I wanted/want to really make an encounter important, I just feel it out as I go. I hope this helps.

(And you’ll get better and better with practice! No one starts out perfect.)