How do I list all layers in an AutoCAD drawing with the number of objects on each?

What cadssistant does

1. Walks the layer table with tblnext so every layer is listed, including the empty ones.
2. Selects every entity with (ssget "_X") and counts them by layer in the same pass.
3. Replies with one table: layer and objects (colour and state on request). Layers with zero objects are listed too, which is the list PURGE would remove.

The same task in AutoLISP

(progn
  (setq ss (ssget "_X") i 0 tally nil)
  (if ss
    (repeat (sslength ss)
      (setq lay (cdr (assoc 8 (entget (ssname ss i)))) i (1+ i))
      (setq tally
        (if (assoc lay tally)
          (subst (cons lay (1+ (cdr (assoc lay tally)))) (assoc lay tally) tally)
          (cons (cons lay 1) tally)))))
  (setq def (tblnext "LAYER" T))
  (while def
    (if (not (assoc (cdr (assoc 2 def)) tally))
      (setq tally (cons (cons (cdr (assoc 2 def)) 0) tally)))
    (setq def (tblnext "LAYER")))
  tally)

Counts entities by layer, then adds the layers nothing sits on with a count of 0.

Good to know

• Entities inside block definitions are not counted: a layer used only inside a block shows 0 here, but PURGE will still refuse to remove it.
• Say "model space only" to add (410 . "Model") to the filter and leave the layouts out.

More in count and measure

• How do I count all the circles in an AutoCAD drawing?
• How do I count blocks by name in AutoCAD?
• How do I get the total length of all lines and polylines on a layer in AutoCAD?
• How do I get the area of every closed polyline in an AutoCAD drawing?