Most countries offer both, and the choice matters more than the small price difference suggests. A single-entry visa is consumed the moment you enter; leave for any reason and you cannot come back on it, even with days of validity remaining. A multiple-entry visa lets you exit and return freely during its validity, subject to the per-visit stay limit.
The classic trap is the side trip. Travelers base themselves in one country, hop to a neighbor for a weekend, and only at re-entry discover their single-entry visa is spent. Popular regional circuits make this mistake common, so if there is any chance you will cross a border and come back, pay for multiple entry from the start.
A few countries soften this with re-entry permits or by treating trips to specific neighboring territories as not breaking the single entry, but these exceptions are narrow and country-specific. Check the official visa description for exactly how entries are counted before finalizing your itinerary.