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In principle you can enter the Schengen countries once on a single entry visa but travel from country to country in the zone.
In practice you should follow the plans you presented when you applied for your visa. Small changes are acceptable but if your travel is very different from what you said you planned to do they may not give you further visas.
When you get a multiple entry visa, the first travel should be as planned, later ones are more free, as long as you keep to the time limits in the visa.