Manure is good for the garden but how much is too much?

Depends on the manure. Many fresh manures will burn plants. If you are using composted or manure that is well rotted, and normal soil just use a bit as a top dressing. If your soil is really hurting for nutrients (a soil test can tell you) then use more and dig it into the soil.

I used to use horse manure and for awhile I had buffalo manure, they work well but the ammonia content is so high that if you use fresh manure you are going to burn your pants. Place it aside for next year if fresh now. Oh, and be careful about using manure that has been dropped in a stall with hay on the floor because you are going to pick up all kinds of weed seed.

If I can I get the manure in the fall when everything is out of the bed and spread it then. Just sort of enough to cover the ground. I don't mound it up there. By the time spring rolls around it has sort of cooked down and I just dig it around in there, and I'm good to go. As the others stated, too fresh and it usually burns plants. If it is rotted or has sat around a good bit, it is great.