Standard screenpacks do not show 5 slots. You need a 5v5 specific screenpack. Search for "Power Levels 5v5 screenpack" or "MegaTeam Screenpack" on the MUGEN Guild. Copy the .def file over.
The most important part of the 5v5 experience is organizing your roster. You need to edit the select.def file correctly to ensure the slots align with the patch's requirements. mugen 5v5 patch
The Ultimate Guide to the MUGEN 5v5 Patch: Expanding Your Roster Battles Standard screenpacks do not show 5 slots
: Allows for 3v3 or 4v4 simultaneous matches (up to 8 characters on screen). mugen 5v5 patch