I have 4 (not counting the 100 free ones that I flip through now and then to delete ones I know I don't want to read).
I have had my Kobo for less than a week, and read one library book, and about 70% through another one.
I read one at a time (1 on my Kindle, 1 on my Kobo that is
Usually read Kobo in the evenings (no cover yet) and take the Kindle with me.
When I first got my Kindle at the beginning of the year I downloaded scads of samples, and was reading quite a few at the same time. Didn't work out so well. I had trouble getting back into one, remembering anything about it!
Now I just read one at a time (although one on each reader).
I have discovered some great writers that I never would have without getting an ereader. I had my "go to" writers, and seldom branched out like I do now.