Jhoon Rhee, the founder of Americanized Tae Kwon Do, has been known to do 1000 pushups a day in the martial arts community.  This man is in at least his 70’s now.  Some of the greatest warriors out there have pushed themselves that almost make them super human.  Of course, they are not.  But they have disciplined themselves harder than the average person is willing to even think about.  It has inspired me to train harder than the average gym member.  One way I wanted to challenge myself was to improve the repetitions in my pushups, but with a twist.  I had to do them on my knuckles.
Beginner Challenge – 250
If you are new to pushups, or it is one of your weak points, I would recommend pushing yourself to perform 250 pushups before the end of your workout.
Intermediate Challenge – 500
For the intermediate challenge, complete 500 pushups before the end of your workout.
Advanced Challenge – 1000
For the advanced, try and complete 1000 pushups before the end of your workout.
Tips
One tip to help you could be setting a time limit, such as 1 or two minutes per set.  It will help you limit yourself so you do not completely burn out and fail to perform the remaining pushups.
If you do this enough, you may well be on your way to doing as many pushups as Jhoon Rhee.