In addition to the solutions above, the Rhino document class has a method that returns the total object count:
230123_RhinoDocObjectCount_00.gh (3.4 KB)
Any input will trigger the component in this example (e.g. a button press). For an event based trigger one could implement this code, and for a time based trigger one might implement something like this.
See the second component here, which takes a layer filter input (i.e. L):
230123_RhinoDocObjectCount_01.gh (5.1 KB)