HI!
I am doing a baby shower cake at the end of this month. It has to feed 50 with generous slices. .
I am planning to do 2-3 tier round- 6/8/10 or just 8/10, 3 layers each. In addition I need to add a baby monkey fondant topper and not sure the size/height I should use.

Please help!!!!

Those sizes will make a very tall tower of a cake - not my favorite design. It also will make delivery very difficult.
As for the topper - make it to fit on a 5" round cake and it will be just right for any sized tiers you settle on using. Don't make it too tall - probably about 4-5" at most.

To make a cake for 50 "generous" servings, you do not need to make higher tiers.

If you make round tiers 8-10-12" diameter, each tier a total of 4" high (two layers each) you will need the equivalent of 5 cake mixes (16 ounce). Now that makes 60 large dessert sized slices among cake lovers.

The regular height tiers will be easier to handle and stack.