2000 Toyota Celica rpm wont go past 6?

I think you are hitting the rev limiter so you don't blow the engine up. And when the Tachometer says "6" that's 6,000 rpms, revolutions per minute which is also 100 revolutions per second...that's fast!

How come I see other people drive celica that goes to 7800 rpm?

6 grand is probably red line. If you want a 7 grand engine you need to rebuild it with HD racing parts. Your engines power probably peaks at 5 grand or a little higher. You would need cams, head work and intake and exhaust upgrades to get power at 6 grand and higher.