The total mailbox limit is 100, however if a large quantity of LTs are completed at once, tnen the first 20 or so LTs will be listed separately and the rest will be combined into one piece of mail to help prevent mailbox overload. So you should be able to complete 100+ LTs at one time without you or your guildies losing any mail, just make sure not to have too much other mail in your mailbox when you do that.
This is also listed at the top right of your mailbox to make sure you can see how many you have:
I would also recommend not trying to hit the limit just in case there’s extra mail for some members (like uncollected event rewards or gifts from Guild mates. Also, within 12 hours of weekly reset is usually also a bad time as there’s a lot of mail waiting for everyone to collect so if everyone isn’t online some people may miss out.