If you have a Battle.net account and have an access to it, you can keep adding World of Warcraft accounts onto it in the World of Warcraft section on the account page. I don't know if there's a restriction to how many you can create yourself. Each of the accounts start as trial accounts and you can upgrade them to the full version of WoW.

For me I just contacted Blizzard to merge one other account under my main account, they asked twice for my account details (in ticket version). When I was dual-boxing, a GM contacted me and asked me to provide accounts names (e-mail addresses) and asked me if I knew that I'll lose all mounts/achies/etc. on my other account. Well, then he asked me to log off of the other account for a minute and he merged it under my main account. First I thought of just merging that one account but when he asked if there was anything else he could help with, he ended up merging 2 other accounts too without asking for the account details. Now I have 4 accounts under one Battle.net account. It looks kind of fun on the Battle.net account page. I wonder how it'll look with more than 4.

By the way, it was all thanks to the community in here. Now I get to use my cool mounts and titles on all my accounts!
... not to mention that having over 10k achievement points looks way more cool than having 300 or less!