How many users can a paved trail support before it becomes too crowded or over used?
Trails almost never reach a level of maximum use. A trail of a given length can accommodate more people than will generally ever use it. The most accessible or attractive mile (like around a popular lake) can receive more use than the other 99 miles in the system.
The other issue is the perception of trail users. Running into just a couple of parties on a wilderness hike can seem crowded, but a popular waterfront trail like San Diego's Pacific Beach boardwalk swarms with bikes, skates, runners, and strollers all dodging each other. It's a poor "level of service" for commuting, but as an experience it's hugely popular.
People do build wider trails and multiple treads to accommodate the different speeds and types of users, and that's what most municipalities in the Denver metro area seem to be doing. The related issue is managing trails properly for the use they get-- speed limits, encouraging fast cyclists to use the road, center stripes, signing narrow sections and poor sightlines, and enforcing some basic courtesy.
It's a matter of opportunity: where are the best trail opportunities, and where are the gaps in places people would clearly like to walk or bike. Urban trail use and demand seems to be increasing nationwide, along with the opportunities as more trails and connections are made.
The City of San Jose system annual trail count shows that trail use continues to rise. San Jose has done a lot to develop a better system for commuting, but they would probably agree that the actual capacity is well below any peak usage.