Indeed, what Hourences said.
Keep the count as low as possible without alterting the shape too much. And keep an eye upon not to waste polys unnecessary. I just got some model back from a friend who made a cinema chair with a cupholder, where that cupholder had almost 1/3rd of the triangles of the entire model. A waste of ressources, especially given the many instances that be used in the scene.
However in the end you have to measure the entire scene and see what takes up most performance, which often can be a lot of other things as well. For example bad particle effects that use up CPU power without even being visible. Whenever you create something, keep the drawcalls in mind and see how you can make it so, that details are used only when the player really sees them or they’re vital for the scene.