For some reason, the iOS 13,5 counts offloaded AppStore apps towards the 3 sideloaded apps limit when using the free certificate. If you see this message when you try to install AltStore or any other app there is an easy fix to the can’t activate more than 3 apps error message. Previous iOS versions are not affected.
AltStore is one of the still-working tools used to install IPA files on iOS 13. Cydia Impactor isn't working with free certificates to sideload apps and games on your iPhone, iPod, and iPad, but AltStore 1.3 fixes this issue. Use this method to fix the "can’t activate more than 3 apps" when installing AltStore on iOS 13.5.
Offload Unused Apps on iOS 13.5
Offload unused apps is a native iPhone settings option, that automatically deletes apps you don't use after a period of inactivity, or when your device detects it's running low on space. On iOS 13.5 this counts for the 3 sideloaded apps limit.
Step 1. Open the Settings app on your device.
Step 2. Navigate to iTunes & App Stores.
Step 3. Disable "Offload Unused Apps".
Step 4. Open the Home Screen.
Step 5. Remove or install all offloaded apps in the system.
From now on you should have an option to install the apps on your device. Please be aware, that when using the free certificate you have still limit up to 3 apps per device. To install the next IPA you need to deactivate or remove the apps.
How to deactivate apps in AltStore
Since iOS 13.5 you can install up to 3 sideloaded apps simultaneously. To work around this, AltStore lets you "deactivate" any of your apps. Inactive apps don’t count towards the 3 app limit, but can’t be opened until activated again.
Step 1. Open AltStore from the Home Screen on your device.
Step 2. Navigate to the My Apps page.
Step 3. Tap on an app to see available options.
Step 4. Click on the Deactivate button.