The answer, like many answers, is "it depends". Specifically, it depends on the broker, and the type of account you have with the broker. Most brokers will charge you once per transaction, so a commission on the buy, and a commission (and SEC fee in the US) on the sale. However, if you place a Good-til-Canceled (GTC) order, and it's partially filled one day, then partially filled another day, you'll be charged two commissions. There are other brokers (FolioFN comes to mind) that either have trading "windows", where you can make any number of trades within that window, or that have a fixed monthly fee, giving you any (probably with some upper limit) number of trades per month. There are other brokers (Interactive Brokers for example), that charge you the standard commission on buy and another commission and fee on sell, but can refund you some of that commission for making a market in the security, and pay you to borrow the securities.
So the usual answer is "two commissions", but that's not universal.
However, while commissions are important, with discount brokers, you'll find the percent you're paying for commissions is minimal, which losses due to slippage and poor execution can swamp.