not possible
too much hardcoded
this is not even problem with interface like icons buttons amount (but this is also would have been VERY hard to fix)
but the main problem is that amount of 9 is tied everywhere in net code
like you select group of 1 to 9 units game send packet of this size to others and also you can ctrl+1 save control groups, all code working with those is strongly harcoded to the many arrays of 9*4 bytes size and they all used and REused in many places of code
so you cannot just change like one number 9 in 1 place
there will be like hundreds or even THOUSANDS of lines of code that harcoded to this size
and the main problems is not even changing them but
to FIND them ALL, that is what is VERY hard
and you cannot afford to miss it
if you miss even 1 its possibly can actually change nothing but also possibly this 1 place can cause some very cursed bugs which will be unknown what is even happening