In most cases I suggest that you follow the manufacturer’s recommendations for how much seed to spread for best results.
The type of grass seed you buy will change this slightly. For example, some grass seeds you purchase will have seed coating, fertilizer and mulch in with the seed (so your 25 pound bag of grass seed may only contain 12-14 pounds of actual seed). Others have 100% seed that is very fresh and most of it will germinate without issue.
It’s always important to read the label and understand what you’re buying and spreading on your lawn. That bag of seed from the box store may be half the price … but it also may be half the seed.
With all of that said, in my experience most homeowners want a thick, full lawn right away after seeding, and may not have the patience to wait for seedlings to fill in over a couple of seasons.
I have always found that seeding a little on the heavy side produces the best results, because not all the seed will germinate, some will be eaten by birds and critters, some may wash away, etc.
The seeding rate I suggest accounts for some of these factors, and should deliver good results for you.