You may register as many as you want. As a matter of fact it's to your benefit to register as many variations of your domain name as possible. Try to register every variation of your domain name especially if your domain name is easily misspelled. Yes, very much so. The more practical and useful it is for others, the more it will be worth.
Domain names can be worth even more in non-monetary terms.
Companies that have spent a great deal of time and money on establishing a corporate identity may find the value of their domain name immeasurable. The worth of a domain name is high, and there is no limit to just how high its value could be. Just remember to protect yourself by making the small investment in registering all variations of your domain. Nowadays anyone can register a .net or .org without proof of being a network or having NPO status. As stated earlier, we highly recommend protecting yourself by registering every variation of your domain name.