Assumptions:

Not considering intra-city travellers. For ex: Thane to Churchgate is not considered as travel for this calculation.

29 states + 7 union territories ~ 30 states

Ignoring transit travellers (People waiting for layover flights etc.)

Ignoring special travellers like Military, Government officials for duty etc.

In India, not too many states have multiple airports that are far from each other. So intra state flight travel is negligible.

Approach:

Types of mass travels: Interstate, Inter district, Intra district. Modes of transport: Flights, Trains, Buses, Private Transport. Frequency of trasport. Avg no of travellers. Total no. of domestic travellers.

International travellers to India.  10 mn/year divided by 365 ~ 30000/day

International travellers from India ~ 30000/day

Add it all up.

Estimation:

Interstate flights: Top 10 states have an average of 6 flights per day between themselves. Top 8 & the smaller 20 have average 2 flights/day between themselves.  The 20 smaller states have average 1 flight/da between themselves. ~ 140 flights/day*200 average no. of passengers = 28,000

Interstate Trains: Top 10 –> average 3 trains per day. Between top10 & smaller 20 –> 2 /day. Between smaller 20 –> 1/day. ~80trains*avg of 1500 passengers = 120,000

Interstate Buses: Each state will have buses to an average of 6 neighbouring states. Each interstate segment is covered by an average of 3govt and 3 private buses. 30 states * 6 buses * 60 avg passengers = 10,800

Interdistrict trains: Average of 25-26 districts per state. Top 6 districts * avg. 1 train per day + 20 districts *  avg. 3 trains per week (3/7 per day) = 15 trains per day * 1500 = 22500*30 states = 675,000

Interdistrict buses: 6 districts * 6 per day + 20 districts * 2 per day = 76 buses * 60 passengers = 4560 * 30 states ~ 135,000

Intra district buses: Average of 4 taluks/district*5 buses/day*60 passengers*30districts*30 states ~ 1,000,000

Summing it all up. ~ 2,000,000 travellers /day using mass transport + around 10% of this in private transport vehicles ~ 2,200,000 travellers/day in India