As many as you want but you can only access two accounts at the same time. Meaning while you are running around you can have an alt from another account following you around gather tears and leveling with you. Or your alt can be your merchant alt but we need all alts to be leveling over time and cannot stay at lvl 1.

Summary:

1 main Account and 1 Alt Account can be used at the same time no more
-Breaking this rule can result in account wipe of all items, temp ban with clearing until you come to the admin, or perm ban.