The answer for how much Osmocote Plus for an 8 inch pot is the same as the amount for a two gallon pot. That would be 4 level spoons of Osmocote for your 8 inch pot/2 gallon pot.  The spoon provided with the product holds about 1/2 an ounce so 4 spoons is two ounces. An easy way to calculate is to remember that a one gallon pot needs one ounce or two level spoons of Osmocote. So with my smaller pots that are half the size of a gallon pot I use one spoon (1/2 oz).  With a pot like yours that is  twice as large as a gallon pot I double the dose for a gallon pot and use 4 spoons. The best part about a slow release plant food is that the dose is forgiving. You will not harm the plants if you use a bit too much or a bit too little. When I plant a new container I will use an empty one gallon pot to fill it up with potting soil and I count how many gallons or potfuls it takes to fill up the container. That gives me the number of gallons that the container holds and because I know the dose is 2 spoons or 1/2 ounce per gallon, the number of gallons times 2 gives me the number of spoons to add. Thanks for asking a great question and Keep Growing, Marianne Binetti