20,000 equals two regiments of irregular troops ... invasion for conquest and all the loot they can plunder ...
More than that. 20K is the size of a well-equipped US division or 4 Roman legions.
Enough of a group that could easily be handled by squadron of B-52s (not the musical group).
What is the difference between a battalion and a regiment?
In modern times, the echelon that falls between Company and Brigade is the Battalion, usually commanded by a Lieutenant Colonel, and Regiments are mostly historic and ceremonial designations. Sometimes Regiment is still used in the name when another unit type would be inappropriate for one reason or another.
A battalion is typically 800 troops, and a regiment may be composed of two of those. A brigade is two or more battalions, which can be combined into a division. A division is an organization composed of around 20,000 troops.
A division is a serious incursion.