There is not a set rule or restriction that limits the number of times a visitor with a valid US visit visa can visit the US. You can visit the US as many times as you desire - of course - as long as the visa is valid. However, the IMPORTANT thing is to show that you were here or are here for a valid reason, which is to purely visit (including attending seminars, business meetings, school conferences, visiting friends/famiy etc) and not work......make sure to keep all documentations, receipts/confirmations of reservations, invite letters (if applicable), financial documents, college documents etc - all relevant documentations - available at the POE and be ready to explain the purpose of your visit at present (and purposes of the visits you'd made) and you should be fine.

Good Luck.