I don't know MC's implementation details, but...
It appears that at any time all thumbnails are displayed with the same value of:
Effective Column Width = Thumbnail Width + Spacing (sum controlled by the slider)
That is, the thumbnails are neatly aligned into columns, independent of the underlying thumbnail's stored image size, so the relevant number here is the Effective Column Width. I assume those numbers are available internally in real time? If so, as the slider moves, calculate and display in real time a rounded integer number:
#Thumbnails = (Window Width)/(Effective Column Width)
Window Width = full width of the current MC window (pixels), presumably obtainable from an operating system query, and includes both panels and view area, minus MC window borders. With MC panels collapsed, #Thumbnails = number of visible thumbnails per row. If panels are displayed, #Thumbnails >= number of actual visible thumbnails in a row. Nevertheless the number is valuable in terms of setting reproducibility and slider position feedback. The user can always set #Thumbnails with the MC window maximized to full screen, in which case it provides a way to manually normalize the number of thumbnails per row across different physical displays on the network (server & clients).
Alternate implementation: Always set Window Width = full screen monitor width
In all respects, the current slider behavior remains the same as it is today. I am just asking for quantitative feedback in response to slider motion (the number disappears when not hovering over the slider). In the underlying code, #Thumbnails is a "derived or reactive" number, not a "controlling parameter." In this regard, the thread title may be a little misleading.
MC 29.0.74 Windows 10/11 (64-bit)