What is the number of negtive charges on a Phosphate group when the phosphorous has a single bond to carbon?

Answer 1

If the phosphorus is directly bonded to carbon, it isn't phosphate, it's phosphonate, derived from a phosphonic acid

R-P(O)(OH)2

where R- is an organic group, and the (O) in brackets is double bonded to P. So you only have two acidic OH groups, and the most highly charged anion is R-PO3 2-.