This is the number of Domain aliases you are allowed on your domain.

Domain Aliases are additional domain names associated with your main web site. So if your primary web site is www.yourdomain.com then an alias might be www.yourdomain1.com and would actually point to www.yourdomain.com. 

Domain aliases also forward e-mail from the domain alias to the corresponding account on the primary domain. So, if you have an e-mail account set up on your main domain called joe@yourdomain.com then when someone sends and e-mail to your alias, which would be joe@yourdomain1.com, it would automatically be sent to joe@yourdomain.com.

You must register each domain name you wish to use as an alias with a domain registrar before you can use it.