The very same day I wrote the original post above I came back and wrote a reply.
Since WordPress now uses infinite-scrolling (it didn't when I last blogged with WordPress six years ago) I wanted to set it for a high number to help fill in the sidebar. The "Blogs I Like" widget allows 50 so I was going to set 50 for the "Posts I Like" widget as well. The "Posts I Like" widget didn't accept 50 so I tried 20 and that didn't work either. I found out through several attempts that apparently 15 was the highest it could be, so I set it at 15. So yes I have 15 now.
If we could ever set it to 50 I would like that.
The "Blogs I Like" widget actually says 50 is the limit for that widget but the "Posts I Like" widget didn't mention a limit at all. If 15 is in fact the actual real limit for the "Posts I Like" widget maybe someone at your end could write that in so people will know. That's where the confusion was for me because I tried to set a number that apparently wasn't allowed and it kept ignoring the number I wanted and automatically reset itself to 5.
But if 15 is the true upper limit then I am fine, I have it at 15 and that's what I am getting.
What you mentioned about the posts that don't have a featured image attached - I didn't know about that but I will keep an eye out the next time I "Like" someone's post and see if it gets added or not.
Thanks for replying eurello.