A division-sized mechanized formation tends to have (as a rough rule of thumb that assumes a big Cold War sized division as the base), around 500-600 armored vehicles in it, with the ratio of tanks to APCs/IFVs depending on the exact type of unit, whether it’s a tank/armored or motor rifle/mechanized infantry unit.
So, as a very rough artificial measurement for an artificial country in an artificial setting, I can tend to just plop down a number of overall tanks and divvy them up.
The next part is figuring out how quickly those formations of tanks would get destroyed in actual fighting. It depends on the kind of fighting and opponent, and ranges from “a year” to “less than a day”.
Coiler's Creative Corner 
I like this little experiment you’re setting up. A lot depends on location, terrain, and such. Is the fighting taking place in built up areas? Is your opponent a peer? What is the enemy doctrine, tactics, etc. If I can help you out in any way, let me know please.
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