The decade counter has a count module 10, i.e. such a counter goes through 10 different states. Therefore, one decade counter allows you to divide the input frequency by 10.
To convert the input frequency of 10 MHz = 10,000,000 Hz to a frequency of 100 Hz, it is necessary to divide the input frequency by 10 MHz / 100 Hz=100,000 = 10 * 10 * 10 * 10 * 10 i.e. it is necessary to divide 5 times by 10. For such conversion 5 decade counters are required.