The manual says (on page 70 - Inserting Clefs, Keys, or Time Signatures)
You can insert clef, key, or time signature changes anywhere in the score.
(Cubase_Pro_Score_12_Score_Layout_and_Printing_en.pdf - WebHelp version)
You can make a clef, key, or time signature change anywhere in the score.
1. Because changes are usually made, not inserted.
2. Because when a user inserts a new clef, key, or time signature—at any one location in the score—, that single insert action is a singular action (i.e. a singular “clef/key/time signature” change).
The score can contain any number of clef, key, or time signature changes.
You can make as many clef, key, or time signature changes as you want, anywhere in the score