For me it depends on a lot of factors, how much air do I have, much is the Ter worth, is it a vital Ter to have, etc.
I’ll usually go 2 inf with air (vs. 1 inf) in most cases. But if there are 2 defending inf and I have overwhelming air (3-4 planes), I’ll match and go 2 vs. 2. With less air I may go 3 inf vs. 2, and for things that may not be that important but I’d like to try I’ll match and go 1 inf + planes vs. 1 inf.
In the case of Russia where planes (either 1 plane or no planes) may be light, I vary anywhere from sending in:
2 inf, 1 inf + 1 rt, 2 inf + 1 rt, 3 inf
These would be for the situation when attacking only 1 inf.
I try to preserve my rt and arm so I use Inf attacks only when possible. The odds aren’t that much different.
Win % (attacking 1 inf)
2 inf = 67%
1 inf, 1 rt = 87%
2 inf, 1 rt = 97%
3 inf = 90%
So unless it is a ter that I must take to maintain the deadzone and prevent the other side from being able to land planes for added defense if they choose to try and stack, I’ll use inf only attacks when I can.