How can I calculate the number of Mondays and Thursdays(Can be any two days of the week) between start date and end date given by the user?

It is directly not possible to achieve this.

A workaround would be to use our Date Differences widget: https://widgets.jotform.com/widget/dates_difference. In the widget settings, you can exclude any specific day and show only Monday or Thursday.

The only downside of the widget is you can't see the difference in the form. Only when you receive it through email or on the form submissions page.

You could also check out the workaround suggested by our colleague here: https://www.jotform.com/answers/1092433-Calculate-the-number-of-business-days-between-two-dates-