create a main driver than repeatedly calls collatz() and determines the length of the returned list.

def collatz(number):
    if number % 2 == 0:
        number = number // 2
        print(number)
        return number
    elif number % 2 == 1:
        number = 3 * number + 1
        print(number)
        return number 

def main():
    number = int(input('Enter a number: '))
    print(collatz(number))

main()