I have only one quilting project, plus generally a scrappy leaders/enders project, going on at any time. There is also one handwork project at a time. This method works for me, partly because I have a small part of the dining room for sewing and need to keep things organized there.
I only allow myself to buy fabric for the current project and the next one, I don't intend to keep a stash though of course there is leftover fabric from previous projects. Not buying additional fabric can be one key to not starting more projects, assuming that you don't already have a large stash since one cannot cut what they do not own. There is a nice thread of people on fabric buying moratoriums if you would like support to not buy extraneous fabric.