This seems to be an unintended side effect of making the player list a child of RscListNBox, instead of the old RscListbox. when calling lbsize on a RscListNBox the actual output is rows * columns. This can most easily be fixed by updating the display script to take the columns into account.

a3\ui_f\scripts\gui\rscdisplaymultiplayersetup.sqf (lines 23-30)

_display displayaddeventhandler
[
	"mousemoving",
	"
		_display = _this select 0;
		(_display displayctrl 1015) ctrlsettext str (lbsize (_display displayctrl 114));	
	"
];

Should look more like (3 is hardcoded here, but we can probably find a better way to pull the column count):

_display displayaddeventhandler
[
	"mousemoving",
	"
		_display = _this select 0;
		(_display displayctrl 1015) ctrlsettext  (str ((lbSize (_display displayctrl 114)) / 3));	
	"
];

This can be fixed (while waiting for this to be patched) on the client's mod side example:
https://github.com/BourbonWarfare/POTATO/tree/master/addons/fixPlayerCount
The provided example has CBA/ACE dependencies due to using some of their macros, but if you want to put together a simpler fix you can just use a config tweak to have the RscDisplayMultiplayerSetup display load your own custom script that has the correct values.

Can confirm, player count displaying properly on dev build 1.65.138842 . It looks like it was fixed by using lnbSize (good solution, for some reason I couldn't get that to work when I tried).

It also appears to be resolved in version 1.66.139279 RC1 (11-11-2016).

This issue is not present in version 1.66.139494 (stable / main branch).