How many cosmic rays were there per cubic centimeter per second through the chamber? If you multiply the number you counted by 20, that's the number of vertically traveling particles, so the total will be approximately 20x the number you actually count. Most (but not all) are muons. You can look at other videos too. The results depend on the altitude, because the particles are absorbed by air, and on the efficiency of the chambers that depends on their design. Enter only the number, and since it is events per cubic centimeter, it will be small. We allow a wide range given the difficulty of accurately counting the rays and the approximation allowing for unseen ones