Even without accounting for the OS the limit will probably be way lower, on my Pi btsync needs 40 to 50 MB of memory to sync 29k files. Furthermore the 512 MB of memory of the Pi B or B+ can't be wholy used by system and/or user processes, a fraction of it has to be dedicated to the GPU (which is required in the boot process). The minimum is supposed to be 16 MB but I had trouble with values lower than 32 MB.