Hi, welcome to the forum!
There are a few ways you could do this.
The most direct would be to use auto-dj with a filter to the effect of "play count is less than..." However, that requires an absolute value so you'd have to decide where to set it, and adjust it as the number of matching tracks dropped.
A more dynamic way (which is what I've done) is to create an autoplaylist. Point it at your library or whatever source you want and limit it to some number of tracks, maybe 5-10% of your library, and tell it to select by "least often played" (i.e. lowest play count). Put any other filters on it you want, or you can have the main rule be "any field has a value" if you don't want any filters. You can play the playlist directly, or even better - set it as your auto-dj source. Then you'll get a contiuous feed of the least played tracks in your library.