The amount of Mother Culture used determines the rate of acidification during a cheese make. More MC in the beginning means that there are more bacteria that will produce lactic acid more quickly. Less MC means that acidification will happen slower throughout the make. By adjusting the amount of MC used, you control not only the acid throughout the make, but continued acidification during pressing, and the ultimate pH of the finished cheese.
The exact amount is not critical but a good starting point is around 1% by volume. For example a 2 gallon make is 256 ounces. 1% of 256 is 2.56. So you would use around 2.5 ounces of MC. You can adjust this to fit your make habits, the type of cheese, and even your schedule. If you want a cheese to finish quicker, use more MC. I prefer a slow and steady make, so we generally use just 1/4-1/2% depending on the cheese.