90 Menu Item Limit in WordPress Menu
Can’t add another item in an already stacked WordPress menu past the point of 90 items? When you save the menu after adding so many items they don’t save and the menu theme location is also lost!
I guess there shouldn’t be that many items in a menu in the first place, but sometimes it may be a necessity. This is not a WordPress issue as such it is more a hosting/PHP resources problem. The issue is a function in PHP called max_input_vars and its default value is 1000, by changing this value to 2000 normal service is resumed. The value is changed in your php.ini file.
max_input_vars 2000
Just add on a new line or edit if existing.