I don't think there's any set number for what's reasonable. Personally I try to only keep books in my TBR list that I know I'd be interested in reading at some point, even if that point isn't right now. When I first got into e-books I downloaded a bunch of free classics that I felt I should read, but I've since gone through my Calibre library and pruned out those books I know I'd never actually get around to reading.
Beyond that, I have no problems having way more books on my TBR list than I could ever read in my lifetime, as long as there's a reasonably high probability that I may choose any one title from it.
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