You're allowed to have as many fire sticks on one account as you want too.
But there's a limit to the amount of Fire Sticks that you can stream from one account at one time.
The amount of Fire Sticks you're allowed to stream from one account will depend on the service you're streaming the content from.
For Amazon Prime you're allowed to stream only two Fire Sticks at one time per account.
For Hulu, Netflix and HBO you're limited to streaming only one Fire Stick Device per account at one time.