Many times over.

Volume of the Grand Canyon is 5.45 trillion cubic yards, more or less. (Figure acquired off the internet and probably reasonably accurate.)

The volume of the average person is less than one cubic yard. Maybe even 1/3 cubic yard. So you could fit between 1000 and 3000 times the current population of the Earth into the Grand Canyon.

But let’s get a bit fancier here. The average depth of the GC is one mile. Average width is 10 miles.

Unless I’ve screwed up the math, there are 5,451,776,000 cubic yards in a cubic mile. So you could cram all 8 billion people in the world in a section of the Grand Canyon less than 1/10 mile long. Probably a lot less if you allow for squishage.