This achievement is a freebie, you do want your army to be at maximum early in the game so buying up to maximum is one of the first things you should do otherwise you will have a hard time finding a weaker country to engage in war with or defend yourself against larger countries.
You can find the force limit number under the Military Tab. For example France start with 30/40 which mean they need to buy another 10 armies to reach maximum value.
Small countries has lower maximum and can easier claim this, however no country will have problem reaching it's maximum if you buy cheap infantry rather then cavalry.