With that many leaves, you can add a lot of grounds before you need to start worrying that you have too many. I usually aim for about a 50-50 mix by volume, but I just guess at it.

If the pile isn't stinking, you don't have too many grounds.

Ditto.

Coffee grounds have a C:N ratio of about 20:1 while tree leaves have a C:N ratio of between 40:1 and 80:1 depending on how fresh they are, but you should pay more attention to what is happening in your compost pile than the numbers adn if your compost pile needs more coffee grounds to keep digesting add them, if things are pretty well digested and you do not need to add more, don't.

Since you have a tarp, I'd cover the pile to better control the amount of rain. Too much water will wash out good stuff.

I'd quit when i ran out of grounds.

I've made compost piles that have only had coffee grounds and those piles turned out O.k., in fact as long as your pile is at least a 80/20 mix (c/n) it should be alright.
In a large commercial pile the C/N ratio is important, but I don't worry about the lack of nitrogen (or too much, within reason)in a small home pile.
ialbtc