I prefer to add one Armored Corps per Army: 4 Infantry Corps and one Armored Corps. That Armored Corps typically consists of 2-3 Armor divisions and 2-3 Motorized divisions. Note that you really only need ONE Armor brigade in an Armor division, which provides the piercing and armor stats to the whole division. MOT or other fast supporting units provide the bulk firepower and defensiveness for a lot less IC expense.
If I'm making an encirclement, it's generally against an obstacle on one side, or between two different Armies. In the first case, I'll drive deep, then turn in against the coastline or neutral country, cutting off the enemy. In the second case, I'll use the armored corps from each of two adjacent armies to drive independent breaches through the enemy line around 3-6 provinces apart, and then meet up somewhere behind the enemy. One Corps is generally sufficient to create the encirclement, but I usually have to reinforce it with regular infantry to prevent the enemy from breaking the ring with desperate attacks from both inside and out. My infantry along the original line can often attack the counter-attackers and potential escapees within the ring, but the ring needs to be strong enough to withstand those combined attacks long enough for you to break the enemy units within. Armor is great for forcing the encirclement, but rather expensive for holding it, particularly if the enemy is equipped with AT brigades in most divisions.
My preferred combinations for mobile divisions are usually ARM+2xMOT+AC or SPART, and 2xMOT+SPART+AC. Armored Cars aren't the most powerful thing on the field, but they provide a substantial movement bonus to the entire Corps in some types of terrain, which can be critical for closing an encirclement or overrunning a broken unit, and they're fairly cheap for what they deliver. I may substitute an ENG brigade in place of one AC per Corps, for river crossings and attacks into bad terrain such as cities or dense woods, rather than waiting until conventional Infantry units (which also get one ENG brigade per Corps) get there. I'm a huge fan of ENG as a penalty reducer in key situations, but they're ONLY effective in certain terrain types (urban, forts, rivers, heavy woods), otherwise they're one of the weakest things you can add to a division (AT is about equal, EXCEPT against armor).
Of course, turning all of that over to AI control means that it will put your MTN troops in the open fields, send your armor to attack a mountain, and launch a fast attack to secure a key province using GAR, while your highly trained marines and paratroopers protect your cities from partisans.