How do I calculate reasonable inode limit?

Inode limit can be considered as a number of files and directories one customer can own. Default WordPress installation has approximately 1,100 files/directories, default Magento install is 10,000, you would need to consider at least 30,000 to be an inode limit for a minimal package for your customers. You can calculate limits basing on current usage of your customers by running the following command. It will list top 50 entries that use inodes the most:

repquota -a | grep -v root | awk '{print $6" "$1 }' | sort -g | tail -50

Do not forget - temporary files and session files are also counted towards the inode limit.