Hi Pauline
I would suggest you to add a counter IC (maybe a 4017 decade counter or so) just after the 555 timer part. 555 timer's output would serve as a clock for this counter IC. Now when you clap, 555 with generate a high pulse for a certain time period and then go low at its output, this will act as one clock cycle for counter IC and it would increment its output by one, each time you clap. I would recommend you to go through 4017 IC'c datasheet and understand how it can be used for the same.
Good luck