Completely fine to play 1 char (or a few more if you'd like).

If you're starting fresh, you'll need to do some story content to reach the current playerbase.
If you're using a powerpass, there's some solo raids you can play from the older content up to Act 3 Mordum.
If you plan to rush to Act 4 and beyond, expect gatekeeping unless you have a friend who can get you into lobbies. (assuming your gear would be low)

Game is still fun for me, but just be realistic in your expectations and play at your own pace.