QGIS/resources/function_help/json/num_selected

{
  "name": "num_selected",
  "type": "function",
  "groups": ["Record and Attributes"],
  "description": "Returns the number of selected features on a given layer. By default works on the layer on which the expression is evaluated.",
  "arguments": [{
    "arg": "layer",
    "optional": true,
    "default": "current layer",
    "description": "The layer (or its id or name) on which the selection will be checked."
  }],
  "examples": [{
    "expression": "num_selected()",
    "returns": "The number of selected features on the current layer."
  }, {
    "expression": "num_selected('streets')",
    "returns": "The number of selected features on the layer streets"
  }],
  "tags": ["evaluated", "selected", "works", "default", "features"]
}