The number of files, which can be added to an archive, depends upon the amount of available memory and the length of file names. Roughly 128 bytes of memory are required per archived file for RAR archive. For example, it can take about 128 MB to compress one million of files. WinRAR has been tested to handle over one million of files.
The size of a RAR archive, as well as the size of any single file within a RAR archive, is limited to 8,589,934,591 GB (9,223,372,036,854,775,807 bytes). Note that to create archives larger than 4 GB, you need to use NTFS, as older file systems do not support such large files.
The size of a ZIP archive, as well as the size of any single file within a ZIP archive, is limited to 2 GB.
In general RAR archive format is much better optimized for heavy tasks involving huge number of files and gigabytes of disk space.