[2021] How many legendary tasks can the mailbox hold?

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.