Three, four, five, six, seven, eight, nine, ten, eleven, twelve, and all integers with a higher value are all technically plurals of 1 as well, including the decimal digits in between each integer, such as 1.4, 5.63, 12.72965, etc.
So, by that logic, there are an infinite amount of plurals for one. Now the real question is: Are there infinite plurals for sheep?