A short-stay Schengen visa allows you to stay in the Schengen area for a maximum of 90 days (within a period of 180 days). You always get a visa for a certain period. It can also be shorter than 90 days. As long as your visa is valid, you can travel freely between the 26 Schengen countries.
Change Schengen visa from single-entry to multiple-entry
Do you have a Schengen visa? And do you want to enter the Schengen area more than once? Then in some situations you can change your visa from single entry to multiple entry.