Memory is not the issue; I'm wondering if there's a limit to the number of files I can put in the .gpx directory. (Yes, I know there are 3rd party apps that will merge them--that's another issue.)
The limits vary depending on which Garmin unit you're talking about. The limits are restricting the number of files and the content.
Examples: https://www.navigation-professionell.de/en/how-many-data-garmin-handhelds-wearables/
Hans