Is there a limitation of the number of (circular/rectangular) dampers installed close to each other?

Yes. For rectangular dampers the minimal distances apply for maximum two dampers vertically OR two dampers horizontally OR a cluster of 2 by 2 dampers (maximum of 4 dampers). For circular dampers the minimal distances apply for up to 3 dampers horizontally OR vertically, with up to one more damper resp. underneath or next to it (maximum 3 + 1 or 4 dampers).