I want to mark seams in the shape of a circle on an object with triangles. How can I do that without selecting every edge (bc that would take too long)?
Things can get a little faster with Pick Shortest Path: select edge, hold Ctrl and select some other edge farther along the seam. Blender should attempt to find a shortest path. Though it’s not always obvious which one would be it, so a bit of trial and error is unavoidable.
but the mesh has only triangles and is only selecting one edge
Triangles should not matter at all. Why would it select only one edge though, I’ve no idea…
Here’s page from manual: https://docs.blender.org/manual/en/latest/modeling/meshes/selecting/linked.html
(It also says you can Tag Seam when selecting as well)
its selecting several edges, but I want to select multple edges at once, e.g. in the shape of a circle. I want it trying to select certain shapes instead of clicking each edge/lines
well, beats me… sorry 
Not sure it’s even possible. How would Blender decide what shape looks like? Even “Tris to Quads” often leads to undesired solutions.
oh. Thought there is maybe an addon for that
lot’s of tools in bl need quads
so with only tris it is not easy !
one way would be to rotate ob so that you can see your arm in 3D viewport then use the Bisect tool to cut across then select only these verts
happy bl