Rather than a limit on the number of messages, Thunderbird may have problems if Inbox grows too big in size and/or, in the case of POP accounts, the file used to keep track of which messages have already been downloaded may develop problems as well.

I would break that down into folders at the server using webmail and move smaller chunks into the server Inbox, so Thunderbird doesn't have to try to download it all at once. After each download, move the messages out of Inbox in Thunderbird, then compact the folder, to make room for the next chunk.

An Inbox with 100k messages is not too big for TB, but your ISP might have a limit on the amount of downloads per day as e.g. with Gmail.