earth pit v/s MVA rating

There is not clear relation between (No. of ) earth pits (electrodes) and MVA rating.

Earth pit is just for inspection and doing test and inspection of earthing electrodes and overall earth grid resistance.

First of all you should know the recommended value of earth resistance by your client or national codes. For example Max. earth resistance shall be 5 Ohms or 2 Ohms.

Then by use of earth resistivity it is easy to calculate the number of earhing electrodes, earth wells or earth grid total length.

Dear friend, please read the IEEE 80 and find your answer.