Number one, the worst that would happen is that calibre crashed. Crashing the OS is somewhat more unlikely.
(You can do it though -- if your %TEMP% is on a ramdisk, say, which is the case on linux, and you try to convert hundreds of books, they all unpack at once and then you run out of RAM unusually fast, making it impossible to do anything without rebooting first. Not sure if that counts.)
Number two, calibre is not going to crash from having a lot of books. Once you hit the hundreds of thousands, you may find however that certain things slow down.
And it really matters how big the metadata.db file is, more than the accumulated sizes of the books.
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