How to know if foetus is moving less? Not meant to lay on your back while pregnant?

So I recently was reading a post on here, and there were some comments around going to the ER (emergency department at hospital) if unborn baby is moving less.

I’m 20wk+4 and have started feeling baby moving around and kicking in the past week. It’s my first pregnancy, and the placenta is in a spot that means kicks/movement isn’t usually felt until around now (according to the docs).

My issue/concern is that baby really only moves around when I’m in bed to sleep, or already asleep. How am I supposed to know if she’s moving less? How do you even count if they move less? How much are they even supposed to move anyway? And at what point do I go to the ER? Like if I think she’s moving a little less one day do I just go, or do I wait a couple of days to see if it’s a pattern, or what?

I also read in a post/comment about not supposed to being laying on your back? The pregnant person had been laying on their back for around 12 hours I think (they were sick), and didn’t know you weren’t meant to lay on your back and was freaked out because their baby had been moving less after that. But none of my medical team even mentioned this to me?

How else do you sleep? Can’t sleep on my tummy. It’s a bit uncomfortable laying on my sides for too long, and even if I do lay on my side and fall asleep I move a lot and usually wake up on my back anyway, am I supposed to tie myself down?

Sorry for the long comment, idk what I’m doing, this is my first (and very likely only due to heart problems) pregnancy, and I’ve been really chill and strangely unemotional about it up until I saw the post/comments about baby moving less and now I’m just freaking out 🙃