To the best of my knowledge, up to 216 apps can be placed in a single folder on the Home Screen.
Please see a screen recording below where I have tested it for 27 apps. I would not like to test for 216 apps because I feel it is counter-productive when you already have the App Library.

After adding 20+ Apps or so Drag other apps to the Folder and hold it there above the folder for a while let the folder open and arrange the apps and it will create space for you additional apps.
If an app appears to be missing from a folder on your Home Screen, try opening the folder and checking the other pages. A folder can contain multiple pages of apps, but only 9 app icons (3 × 3 grid) are visible at a time when viewed from the Home Screen. Swipe sideways within the folder to check whether the app is located on another page.
If you still cannot find the app, it may have been moved to the App Library or another folder.
You can rearrange apps by dragging and dropping them wherever you want on the Home Screen, including moving them into or out of manually created folders.
However, drag-and-drop rearrangement is available for Home Screen folders, not for folders in the App Library.The App Library is automatically organised by iOS into categories, whereas Home Screen folders are created and organised manually by the user.
If the app is in the App Library, you can add it back to the Home Screen by touching and holding its icon and selecting the appropriate option, or by dragging it onto the Home Screen.