Well let's look at it I suppose.
Dice odds
Scenario 1: 20 armies on 1 country
Rounding odds to simplify (favoring attacker).
Attacker kills 11 per 10 rolls
Defender kills 9 per 10 rolls.
Roughly 18 rolls needed to kill 20 defenders.
~16 dead
3 to maintain dice advantage
16 + 3 = 19
Say 20-21 to account for rounding.
Scenario 2: 20 armies on 20 countries
Attacker kills 6.6 per 10 rolls.
Defender kills 3.4 per 10 rolls.
20 rolls minimum needed to win plus 35% will lose and need to be redone.
~9 dead.
3 to maintain dice advantage
18 left behind on fields to occupy.
9 + 3 + 18 = 30
Summary
Scenario 1: 20 armies on 1 country - Willl require less armies to kill
Scenario 2: 20 armies on 20 countries - Although scenario will require more attackers, you will have more left over afterwards. Even assuming they both start out with the 30 at the beginning. They'll just be spread out like your opponent was.