How many fries does it take to cover one stick of butter?

Assuming you mean french fried potatoes, you haven't specified whether
you mean shoestring fries, steak fries, Ore Ida fries, or what.  Each
french fry has its own size and length, thus the answer will vary -
even when the fry is specified, since size of individual fries vary
within a type, you can only hope to achieve an average answer.  Also,
when saying stick of butter, do you mean the average stick found four
to a pack or the single block of butter available, also able to be
referred to as one "stick?"  European or American butter (sizes vary)?

Also, do you mean for the stick to be solid, spread, or melted?  I
assume by "stick" you mean for it to remain in the solid state...

It depends on the exposure and on the enemy's firepower.  On a typical
mission, I would think one or two well-trained fries armed with
assault rifles ought to be able to provide adequate cover.