How many items can fit in a menu?

Hi everyone. I’m using the Appearance -> Menu features a lot and I have encountered a big problem with my main menu, which is the top navigation item of my website.

The menu does not seem to accept more items. If I add one new item, another previous item disappears… so I can never have in the menu all the items I need.

It is a big menu, with maybe 60 or 70 items/subitems… but I didn’t know there was a limit. Anybody knows how to solve this?

Thanks a lot