Exactly.
Most teams want to sign their top ten picks, because if you DON'T sign one of those guys, that slot allotment is subtracted from your bonus pool. While it does happen from time to time it's relatively rare. The Sox draftees who are high schoolers 1-10 SHOULD sign. Mayer, Rodriguez, McEvleny.
After that, you can count on a few HS kids signing, sometimes they take a kid for insurance to see if someone else doesn't sign with the hope that they might be able to scoop them up. Or, to build a relationship etc etc. Sometimes, with some of these kids, they just want to play ball and start their professional careers and it doesn't take much to sign them. Like Brandon Howlett who signed for 180,000 in the 2018 draft 21st round.
I'd think with only 20 rounds, you're going to see teams drafting less and less players they can't sign, but I still think you will see 3-4 guys with most teams drafts who do NOT sign.