limit in the "Custom Thread Fields"

[ZiNgA BuRgA]

If you're referring to assigning forums to thread fields, there is a bit of a limit to how many you can assign.
If you want to assign it to all forums, unselect everything in the forum selector (selecting none = all forums).
MyBB doesn't perform too well with a large number of forums, so the assumption is that you won't be assigning many, but if you really do want to increase the limit, increase the width of the 'forums' column in the 'threadfields' table.