I would say as long as they have swim-room at the front of the tank and plants and toys to hide in, sleep in, or swim through, there's really no over-doing it.
For example, I have 3 large silk plants, 2 medium silk plants, 2 small silk plants, 2 more small silk plants in a wine glass, 1 more small silk plant in a martini glass, a coffee mug, a terracotta flower pot, a leaf hammock, a pile of glass stones with an air tube underneath for small bubbles, 2 medium marimo, 1 small marimo, a floating canvas mesh tunnel, and 1 ornamental decoration in my 10g and my guy has plenty of swim room up front and many things to swim through, hide in, or sleep on.
There's a bit of space between his toys and the back of the tank so he can swim back and forth there as well as in the front.