land stacking filter ON
what is the symbol shown in screenshot
The 12/12 means that 12 stacking points out of the maximum of 12 (hence the red color) are present. Check the rules, you are confusing number of units (here 9) with their cost in stacking points (12, indicated at the bottom on the left part of the unit panel). This is why the filter is called "Stacking", not "Units"...
You could have even less units if they were costing more in stacking (in your example, the logistics regiment and the tanks, with their numerous vehicules, use 5 stacking points for 2 units)