The overview of visa requirements/exemptions for entry into the Federal Republic of Germany shall be checked on the website of the Federal Foreign Office: https://www.auswaertiges-amt.de/en/visa-service/-/231148
Schengen visas may be single-, double- or multiple-entry and allow the holder to spend no more than 90 days in Germany and other Schengen countries within a 180-day period (calculated from the holder’s first arrival in the Schengen area).
The European Commission has a short-stay Schengen visa calculator at https://ec.europa.eu/home-affairs/content/visa-calculator_en, which you can use to calculate whether you fulfil the general 90 days/180-day rule or not, and to estimate for the maximum length of stays in the future from the intended date of entry on the basis of previous entry and exit dates.