it depends on the number of rainy days and also with the average number of rainy days per annum

I agree with you Drlatief Ahmad

well,

The average of rainfall only

For what purpose?

Here's one example from a hydrology point of view: https://en.wikipedia.org/wiki/Water_year

And the UK Met Office has a nice definition and determination for different parts of the world:https://www.metoffice.gov.uk/weather/learn-about/weather/types-of-weather/rain/rainy-seasons

But you need to be careful because definitions may not be precise and just because there is a definition doesn't mean it can't rain or not rain at any time.

And in a time of climate change these things might be changing depending on location.

based on the number of rainy days