Internal format of RAR and ZIP archives created by WinRAR allows to store practically any number of files of practically any size. Actual limits depend on the file system and available memory. Some older file systems do not support files larger than 4 GB.
WinRAR allocates memory to store name and attributes of every archiving file. If quick open information is added, twice more memory is required to store file name and attributes. While concrete amount depends on a file name length in UTF-8 encoding, typically we can just add several hundred bytes per file to memory required for compression dictionary to estimate memory use when archiving. Also please take into account that 32 bit WinRAR cannot use more than 2 GB of memory. If you need to process a lot of files or use large dictionaries, 64 bit WinRAR may be preferable.