I always have my spears in 4 pikes in 6.

But with Shock i usually use 3 though i kow that the longer it is'then it means your unit can wrap around the flanks.

I'm debating wether to use 3 or 2.
I'll need to do experiments later.
It's the same with Cavalry.
Apparently the Longer the front the better'but it means more chance of them getting caught as it makes the unit unwieldy and difficult to control.