If you wanting to print more than 15 word documents all at once by highlighting them all, you can’t. To change this you can edit it in the registry.
Press Windows key and R and type Regedit.
Then either go to HKEY_CURRENT_USER or go to HKEY_LOCAL_MACHINE depending on if you want it to affect the current user or all users. Then go to \Software\Microsoft\Windows\CurrentVersion\Explorer and create a new DWORD (32 Bit) Value. Call the Value MultipleInvokePromptMinimum
Then change the base to Decimal and type the number you would like to be able to select before the menus disappear. Don’t put a massive value in though as you could accidentally open 100+ word documents and crash your machine.