Participating in multiple studies
Dutch research centres that test medicinal products have the rule that volunteers are not allowed to participate in research more than four times a year (once every three months). Research centres that carry out a lot of research with medicinal products using healthy volunteers check this using the VIPCheck (Volunteer Inclusion Period Check). That is a system which precisely records who participated in research and when.
People are not prohibited to participate in multiple studies at the same time. However, this is not the preferred approach because it can have a detrimental effect on your safety and the reliability of the research. Participating in one research study is not allowed to affect another.
In the case of research with a medicinal product you are usually not allowed to participate in multiple studies at the same time. If you do there will be a risk of cross reactions which means that one medicinal product interferes with the working of another.
Sometimes people are obliged to take a break between studies. That is certainly necessary if there is a possibility of the studies affecting each other. How long this break has to be depends on the type of research. You should ask the researcher whether you are allowed to participate in new research (again).