Try a reinstall first ( delete the existing copy first) and report back. I do not know the maximum number of songs it will display in a playlist as opposed to your collection on your NAS ( the two are not the same) but 50 songs seems far too small. As for only playing one song at a time that sounds pretty much like this question from a couple of weeks back:
This is just for starters and I am hoping for an easy result. Otherwise I need more information; Android or IoS? When you select UPnP what are you seeing ( I see my collection of well over 1K albums, not just songs)? What UPnP server software are you using?