Very good question @nonsowrites and you got a lot of good answers as well.
How many comments do I get per post? It depends, sometimes I get 5, sometimes 10 and other times 2 or three. Compared with the number of active users I follow, if my content is of any impact? I hope so, but content is not all.
I've learned that engagement is the key here. You can have the best content and not get any comments. Also you can have like 50 comments if you are running a contest that requires a link in a comment with your participation post. Also you can get a bunch of comments hosting a giveaway. There comments are somehow generated by the giveaway or the contest prize.
If you want real comments, you go out first and engage, comment on people's post and those people, if they are active and well educated, will come and comment on your content. That's how you build a community, a circle of friends and get real, valuable comments.
You can get 10 - 15 meaningless comment at your posts left by spammers who are after some giveaway hosted by an app (allow me not to name any here), comments that mean nothing at all. And you can get 5 - 10 meaningful, constructive comments.
How many people have I seen complaining about not getting any comments, blaming people of being lazy or not care, and when I went to check the user's comments, I see two or three comments left by him/her, usually to the same people. Waiting for comments is not going to bring you any.
So I'd say engage and look at the quality of the comments, regardless of the numbers.