Most visitors to Russia require a single entry visa. A single entry visa is for people who intend to cross the border into Russia only once during their visit.
People who do wish to leave Russia and re-enter during the 30 days from when they initially arrived (for instance, entering Russia then taking a trip to a neighbouring country such as Ukraine, then re-entering Russia once more to continue the journey) will require a double entry visa.
In either case, you will need to make sure that your final exit date from Russia is no longer than 30 days from your initial date of arrival. For example, if you enter Russia on 1 June, you may wish to exit again on 8 June, and then re-enter on 20 June, but you must finally leave Russia by 30 June.