> length of the ID column is 20
This is correct. The IDs are 64-bit numbers or roughly 20 digits long. Of these 64-bits, the first 16 are reserved by Mendix hence the big number that you see. That leaves 48 bits which corresponds to 281,474,976,710,656 possible objects.
This is well beyond what you could theoretically store in a database before running out of space.
The most certain way to give it a go by running a microflow that will add objects until the app crashes.
My second advice is to ask this question to support.mendix.com