You want to count the number of users on the site and increment it by 100? You'd need to use CB Query Field for that and query the _users table to count them. Example as follows.
Code:
SELECT COUNT(*) + 100 FROM `#__users`
If you're just wanting to add 100 to some other field then you'll need CB Code Field for that, which lets you use custom PHP to display as a field. Example as follows.
Code:
return (int) '[cb_users_count]' + 100;