It seems that there is no hard limit with ext4. It depends by inode size, disk size, etc. I searched a little and I didn't get any deeper than that. Anyway, df -i reports this about my 2 TB SSD

     Inodes   IUsed     IFree IUse% Mounted on
  122068992 4724107 117344885    4% /

I guess that 122 M inodes in the first column is the limit of that file system. Much more than 5 M but Google Drive is probably giving a quota to users. No idea of which file system it actually uses on the disks.