You don't provide enough information.

General rule of thumb is to "fill up" your storage group before creating a
new one. Since you can have multiple databases per storage group smaller
implementations won't see the need to go to second storage groups unless you
have some significantly different SLA's you need to provide those users. How
many users you have, what type of mail users they are (heavy, medium,
light), how large the mailboxes and what are your restore SLAs will
determine how you need to set up your storage groups and databases.

In your case below you'll need 2 of those 8 drives to set up a log file
drive. That will leave you 6 drives. Less one for raid parity and you now
have 5. 180GB of empty space. Save room for maintenance and emergency growth
and you'll max out your db size at around 25-30GB per database.