Can I have extension of my B2 visa?

You can apply but that does not mean that you are guaranteed to get the extension granted.

Usually, you can request an additional 6 months after the initial permission period. You were granted 2 months already. This leaves room for 4 months but the extension is usually granted once.

An extension of a B-2 tourist visa is usually granted for good cause shown. But a subsequent extension is a rarity and in your case will likely not be granted. If you have a multiple entries B-2 visa, you should consider returning to your usual country of domicile before the expiry of your current B-2 stay. You can then return to the States in a few months. If CIS knows of your intention to take an examination, it may conclude that you are not a bona fide B-2 tourist and hence, may revoke your right to remain in or to enter the US with a B-2 visa in future.

Sure, you can apply. The question is whether your application is going to be approved (which I doubt, under the circumstances). Perhaps, you might be able to apply for a change of status through your prospective employer. Alternatively, you should consider filing for a change of status to F-1 student visa, especially if you are already taking classes for your dental exam preparation: taking classes while on a B visa is considered a violation of your non-immigrant status; and USCIS might take a dim view of your stay in the U.S. on a B visa for the purposes of taking a professional exam. Otherwise, consider this: your extension application will take 4.5 months (in Vermont Service Center; 2.5 months - if your previous application was processed in California Service Center; look up the address on the notice granting you the extension). Even if it will be denied, *so long as you do not accrue 180 days* of out-of-status presence in the U.S. after expiration of your first extension, you will not become inadmissible (yes, the U.S. consul is likely to give you hard time when you apply for your next visa; but, if you can show that you filed for an extension and remained in the U.S. only until your exam, your overstay will likely be forgiven).

Sure. That doesn't mean they'll approve it, but there is no limitation on B-2 extensions