I just checked MW and will paste below what's there. The pi that we think of in math has the plural 'pis,' but another kind of pi, that can also be spelled 'pie' (not the edible pie), has the plural 'pies.'

OK, here's the scoop of pi from MW:

Main Entry: 4pi
Function: noun
Inflected Form(s): plural pis /'pIz/
Etymology: Middle Greek, from Greek pei, of Semitic origin; akin to Hebrew pE pe
Date: 1823
1 : the 16th letter of the Greek alphabet -- see ALPHABET table
2 a : the symbol ð denoting the ratio of the circumference of a circle to its diameter b : the ratio itself : a transcendental number having a value to eight decimal places of 3.14159265


And:

Main Entry: 1pi
Variant(s): also pie /'pI/
Function: noun
Inflected Form(s): plural pies
Etymology: origin unknown
Date: circa 1659
1 : type that is spilled or mixed
2 : a pi character or matrix


So we have two possible plurals for pi, depending upon meaning. Cool, huh?