Open Question: 300 crunches was too easy what should my new goal be?

Yesterday I was challenged to do this: 300 reverse crunches 300 oblique crunches (each side) 300 normal crunches I did it and it was too easy, I think I could have kept going so instead of 300 I want a new harder goal that'll make a longer workout that I'll have to train up to be able do. What would be a good and really challenging number of reps for my new goal?