How many ground and power pins to have in a connector?
There's a great book from Henry Ott that covers this -- unfortunately I'm on vacation so I can't take a picture of the relevant diagram. The book is Electromagnetic Compatibility Engineering.

Here's some quick points though:


from a DC power point of view, how much current does your device require? Look at the rating of each conductor, de-rate if necessary to give yourself margin and choose accordingly the number of power pins. Remember you need 1:1 power to ground.
from a AC POV, you want to minimize the loop return area of all of your signals. You want each signal to have its own GND / return immediately adjacent -- two schemes to do this:


S G G S G G S G G S G G S (one-two ground per signal, no adjacent signals)

G S S G S S G S S G S S G (one ground per signal still, more efficient utilization of space).

The idea is to minimize crosstalk and radiated emissions. While DC follows the path of least resistance, AC follows the path of least impedance. In this case, providing a return path immediately adjacent to a signal will help minimize the size of the overall current loop, reducing your radiated emissions.

Additionally, are these all single-ended? Differential? Expected signal rate? Expected edge rates?