I found an old function by John Uhden in the Autocad forums and modified it slightly.  I changed a couple of single character variables to meaningful names and I return a value instead of a list containing all the instances of the most common value. 


Test:

Returns:
9.3

Thanks for the suggestions.

From https://www.investopedia.com/terms/m/mode.asp#:~:text=The%20mode%20in%20statistics%20refers,the%20numbers%20in%20the%20set.

The Bottom Line:
In statistics, the mode is the number that occurs most often. A data set can have one or more modes. The mode is different from the mean, which is the average of the numbers in a set; it's also different from the median, which is the midpoint of a set. Finding the mode in a set of numbers can tell you which data points occur most commonly, which can be useful information when analyzing statistics.


Just use count items by Lee Mac

;; Count Items  -  Lee Mac
;; Returns a list of dotted pairs detailing the number of
;; occurrences of each item in a supplied list.

(defun LM:CountItems ( l / c x )
    (if (setq x (car l))
        (progn
            (setq c (length l)
                  l (vl-remove x (cdr l))
            )
            (cons (cons x (- c (length l))) (LM:CountItems l))
        )
    )
)

Examples:

(LM:CountItems '("A" "B" "B" "B" "C" "C" "D" "E" "E" "E" "E"))
=>  (("A" . 1) ("B" . 3) ("C" . 2) ("D" . 1) ("E" . 4))


(LM:CountItems '(1 2 3 3 3 4 5 6 6 7 8 8 8 8))
=>  ((1 . 1) (2 . 1) (3 . 3) (4 . 1) (5 . 1) (6 . 2) (7 . 1) (8 . 4))


Here a version in Python if your feeling adventurous 
https://www.theswamp.org/index.php?topic=59527.msg621035#msg621035

Is opening the drawing, so we have some ambiguity on this definition.

If we are good with opening the db, you would pass in the objectid that represents the block table record for the space
Either modelspace or paperspace   

Isn’t there a function CMLContentSearchPreviews.GetBlockTRThumbnail(btr);
Just pass in model space