From Conquest Wiki

Any unit combination that beats another unit combination (of equal, one lower or one higher power) reliably and with a non-negative power gain, is considered a counter to this other unit combination.

You can read the counter table in the following way: squad vs counter squad => result => power gain.

In Plains

Squad	Counter Squad	Result	Power Gain
1 Power
			0
			1
2 Power
			2
			0
			2
			0
			2
3 Power
			3
			3
			0
			1
			3
			1
			1
			3
			2
			2
			2
			2
			0
4 Power
			2
			2
			0
			1
			1
			2
			0
			1
			4
			4
			2
			4
5 Power
			1