To be read

How many books does it make sense to accumulate on your 'to be read' shelf if you have one, or in your 'to be read' stock if you don't? The question is prompted by Lucy's excess-of-books post over here. Lucy doesn't say how many of her 10,000 [!] books fall into the TBR category but it sounds like there might be quite a few more than the shelf-worth that I allow myself. I don't mean I only have one shelf of not-yet-read books. I have far, far more than that. But many of these unread books are ones I once bought with the intention of reading and things have moved on, some were bought just to have, or with the vague idea that I might one day read them; and so on. My TBR shelf, as named, holds the books I mean to read soon, and I don't permit myself to acquire any more in the same category until a gap opens up that can accommodate them. My TBR books have evolved from being part of one shelf to being the entire shelf, but I will NOT let them grow any further. When I told a friend not long ago that I had 50-plus books queued for reading, she looked at me as if I was caught up by a spirit of excess indeed. Half a dozen, OK; but a whole shelf! Is there any rationality, though, in this matter?