See this: en.wikipedia.org/wiki/List_of_N…
Note, Google Maps uses the new numbers. The new system uses odd numbers for East-West, and even numbers for North-South highways.
E.g. Earlier NH4 ran from Mumbai to Chennai. Now the old NH4 and NH8 have been combined into NH48: Delhi-Mumbai-Chennai. en.wikipedia.org/wiki/National_…
The new numbers are NH48, NH60 (extended from Nashik to Dhule), NH65 (extended from Solapur-Hyderabad to Vijaywada).
The old NH17 (Mumbai-Goa) is now NH66 and runs to Thiruvananthapuram
Note - certain key roads like the Mumbai - Pune Expressway are not part of the National Highway system, and hence don't have a 'NH number'.