I would go about it like this:
Make some lists, one with Layer names, one with abbreviations for the plant names, one with the multipliers [quantity of plants per square meter]:
(setq
layers '("LayerA" "LayerB" "LayerC")
abbs '("AXZ" "BTW" "CPM"); abbreviations for plant names
mults '(3 5 10); multipliers for plant quantity per sq meter
); setq
Select your Polylines, or have the routine select all Polylines. Step through the selection, and for each, make the Polyline a VLA object, stored in a variable [here, plobj]. Then do something like this:
(setq
pos (vl-position (vla-get-Layer plobj) layers); its Layer's position in the list
qua (* (fix (vla-get-Area plobj)) (nth pos mults)); the quantity of plants for that area
str (strcat (itoa qua) " " (nth pos abbrevs)); the text string [rearrange/add to the pieces as desired]
); setq
Depending on your Units settings, you may need to convert the Area property to square meters with a calculation.
You can use the bounding box to find the middle of the extents of the Polyline's area, if that's a good basis for positioning Text or a Leader start point or something. Or you can leave that to the User to pick. You can have different Layers for the notation with another list, or use the Polyline's Layer; in either case, you would set that Layer current for the Text/Leader/whatever.
Consider also testing whether each Polyline is closed, or is on one of the appropriate Layers, etc., before applying the process to it. [A Layer check would be essential if you're having the routine select all Polylines.]
Your use of the word multileader makes me wonder whether you are using one designation of quantity for more than one Polyline area. That would make it more complicated, and would, I assume, require User selection of which Polylines to tag collectively. If you are doing it for each Polyline individually, I would think an ordinary Leader, or even just a piece of Text, would do. If it's just Text, you could have it middle-justified at the midpoint of the bounding box automatically, and look around afterwards for any that need to be repositioned, or you could leave it to the User for each one to designate a location. That would actually be easier to accomplish than a Leader- or Multileader approach, unless you're willing to require all Leaders/Multileaders to be drawn with the same number of defining points.