Later in the game you have a great economy, you’ve taken out some countries that were in your way, got a few universities so you can resurge fast and you got lots of good infantry that have all seen battle. And that is how you pay for so many units. This gives you the chance to resurge things that make your economy the best one around. This means you don’t need to resurge anything related to canons, don’t resurge half of the things that have to do with infantry, and just resurge what you need concerning naval technology. I almost never use canons in the beginning of the campaign. So I use basic cavalry, the first ones you can recruit, 4 units in an army. This takes a lot of units so I need to keep it cheap. If you have a lot of different units it's harder to merge and still have an army with many units containing the maximum number of soldiers. You need to keep units in cities you just conquered so this is not to bad anyway. The units that need the two turns to 'heal' stay behind. So after I merge the units I send the army that is ready to fight on its way. I always send in three or four armies when I need to take down an enemy fast. I keep it as simple as I can, just 2 different types of units, that way I can keep individual unit strength up by merging units after a battle.
