Upvote:1
A national D visa would by required if you want to visit a Schengen state for more than 90 days out of any 180 days. For frequent short trips, the best would be a Schengen C visa with multiple entries and a very long duration.
Long, multiple-entry visa are given if there is a premise to justify them and if the applicant had shorter, possibly single-entry visa before without any problems.
Even if the duration was more than a year, the multi-entry visa would still be subject to the 90/180 rule, it just saves you the hassle of repeated applications. Is is possible to visit many different Schengen states with such a multiple-entry visa as long as it says "Schengen states" or the equivalent in the respective language. The premise for the initial trip needs to be documented in detail with the application. For later trips you should keep the medical insurance in mind, bring enough money to cover your cost of living, have a plan for onward travel, etc.