Parsec can make more than 4 controllers, but the way games behave with Parsec's controllers will vary after 4. The main reason for this is the Xinput controller API used for Xbox controllers, which has an inherent limit of 4 controllers. Games can often fail to properly handle the limit, and they may need you to alter settings or the type of controller used. Here’s what you can do to make more than 4 controllers work correctly, in case it doesn’t.
You may see different behavior by changing whether Steam Input is on or off. by Right-click the game in your Steam library and go to ‘Properties > Controller’ to change.
Cemu's "Xinput" controller API will not let you choose a 5th controller. You need to use a different API for the remaining controllers. WGI Gamepad is a Windows 10+ API that allows more than 4 controllers at the expense of no input while the emulator isn't the active window. SDL works but is buggy. DirectInput works but can be a pain to configure.
There are files for Cemu's WGI Gamepad in the setting up controllers article if you don't want to configure it yourself.
At around Yuzu Early Access 2041, Yuzu has added the ability to allow 8 xbox controllers at once in Emulation > Configure > Controls > Advanced > Enable Xinput 8 player. Enable this and restart the emulator if you want more than 4 controllers at once.
Test your game with more than 4 controllers beforehand to ensure it's not working, and look for in-game settings related to Xinput, DirectInput, 4+ player or 8 player controllers.
If you can't get it to work, games may behave differently if you change the settings on the host to create Dualshock 4 controllers instead of Xbox 360 controllers.