So what do leaves do? They raise the organic matter percentage in the soil, provide fall food for your worms and micro-arthropods living in your soil, encourage bacterial and fungal activity in the soil, and mulch the soil for winter. Lawns that have been leaf mulched will retain their green color further into winter, green up faster in spring, and generally show greater health than lawns that receive no mulching.
How much should you use? Close to 300 pounds per thousand square feet disappears into the grass in a few days if well-chopped, but applications certainly do not have to be made at that rate. This study shows the results of mulching 150 to 450 pounds of leaves into the soil, and effects are generally excellent.
But isn't leaf litter ugly? Well, yes. The above study states that leaf litter at the highest application resulted in leaves still left on the lawn in spring. However, if you have a rotary mower it becomes easy to render the leaves invisible. With the very large amount I'm using, I mow slowly four times over the area to reduce the leaves to bits smaller than the nail on my pinky (and I have small hands!) By the time I complete the task, almost no visible litter remains--99% has fallen into the grass and becomes invisible. The remaining 1% disappears with the next rainfall.
What's the NPK ratio of leaves? About 0.8-0.35-0.15 according to this source, but don't worry about it. Trees extract almost all the chlorophyll and nitrogen and place it in root storage for the winter. Since there is a small amount of phosphorus in the leaves, mulching them is better than allowing rainwater to percolate through them while they sit on the street as the phosphorus enters the waterways and causes algae blooms that aren't good for lakes and streams. The reason leaves are mulched isn't to feed the lawn but instead to provide additional organic material for the soil.
Isn't it more work? No, not unless you import leaves. It's far easier and faster to mulch the leaves your trees drop into the lawn than it is to rake and discard them.
How much at once? To avoid smothering your lawn, try not to chop up more than about an inch of leaf litter at a time. That may mean mowing a bit more often under your trees, but the results are worth it!