For SyncBackSE/Free the maximum number of files depends upon the amount of free memory (RAM) available (and also the amount of memory fragmentation). The following article has details on the memory usage of SyncBackSE:

SyncBackPro, however, is not limited by the amount of free memory (*). It will use memory (RAM) to store file information until memory runs low or the number of files approaches 200,000. At this point it will move the information from memory to an internal database (generally stored on the local drive) and use that. This means there is generally no practical limit to the number of files that can be processed (the limit then becomes the amount of free disk space in the designated area).

SyncBackPro and SyncBackSE are both available in 64-bit versions, which means they can use all available memory. 32-bit versions are limited to 2GB.

(*) Note, however, that this does not apply when using a cloud service other than Amazon S3, Microsoft Azure, Google Storage, Backlaze B2, OpenStack, OVH and Egnyte. Due to the nature of how these cloud services work (Dropbox, OneDrive, Box, SugarSync, etc.) SyncBackPro must keep details of the files in memory and so the amount of free memory becomes the limitation. For more details, please refer to this article: