Regarding the fast on Mondays the more the better but atleast 16 Mondays, As Mercury rules the 10th house and Jupiter is placed in the 10th house you can deals with books, stationary items,pens, pencils, coaching, teaching, religious items, anything related to education, as Mars is in the 10th house you can work on anything related to security, medicines. You can also look at dealership,retail of certain items related to Mercury & Jupiter. Mars is a functional malific in your horoscope, Yes it is Vargottam but its mahadasha will be challenging, health issues can also be there. Prevention will be better than cure. Professionally your initiative will matter the most. Reading Hanuman chalisa will certainly help you during this period.