I've had quite a few this pregnancy. I had 3 in the 1st trimester, just because I've had a higher amount of miscarriages. Had 1 at 20wks which is about normal for everyone. Then I had another at 24wks because he was too wriggly at the 1st and they couldn't everything they needed.
Then when I hit around 30wks they noticed he was measuring further along so had an ultrasound at 31wks, and again at 35, turns out he is a big boy so I will need monitoring every week until birth.
My dr does one for confirmation in 1st trimester. Then one for gender 2nd trimester. An then one around 35 weeks to see position an hiw big the baby is.
At my doctor it's a heartbeat check every 4 weeks until I'm 28 and then every 2 weeks, then every week. But I'm getting an ultrasound done at 20 weeks to see what we are having.
1 in the 1st and 1 in the 2nd...some people get more...just depends on if you're high risk...or what your insurance covers...stuff like that.