In my early years on this earth, there was a debate going on, in which I never joined... it was 'HOW MANY ANGELS CAN DANCE ON THE HEAD OF A PIN?
If anyone can research and find their conclusion to this argument, I will believe that this argument will reach a conclusion too someday.
Why I believe this one is UNANSWERABLE is that of all the kinds of evil spirits, fallen angels are one of them... and the number of total angels is said to be INNUMERABLE... and one third of them fell... so if one third can be multiplied by 3 to get the total number, that would make them NUMERABLE. So, the one third must be innumerable also.
Second, aside from the fallen angel number, we would have to add the demon spirits that come from pre-adamite beings that now are disembodied, not yet bound evil spirits. Since we do not know if their number were millions, billions, or trillions, we are hindered in counting. God leaves the number unrecorded.
Some evil spirits, are part angel, part human, from before the flood, and also after that... giants that died in the flood, giants that died when Israel retook the promised land. These 'ruling powers' as we call them, are also not revealed to us via number...
So, we add all those numbers together, to get all the evil spirits that could be available to attack, subtract those already bound in the abyss, and that is the number... Sort of an uncountable number not worth bending our finite minds to count, and unable to help us in the battle even if we knew... so why worry about it. We have more important things to do, and don't worry...
'one shall chase a thousand, two shall chase ten thousand'... so the millions of Christians can chase the innumerable number... no matter how many it is.