India currently possesses 15 Submarines.
1 Arihant Class Nuclear Submarine.
1 Akula Class Nuclear Submarine.
9 Sindhughosh Class Submarines.
4 Shishumar Class Submarines.
Under construction/in trials-
India currently has 8 more Submarines under construction or in trials.
6 Kalvari Class Submarines.
The first one enters service next month, while the 6th and the final one by 2020–21.
2 more Arihant Class Nuclear Submarines-
First one, the INS Arihant is already active. The second one, INS Aridhaman is in trials and is expected to be ready by 2018 , while the unnamed third one by 2020–21.
So India adds 8 more Submarines in the next 4–5 years till 2020–21, taking the total submarine force to 23.
Future road map-
Next in line is the Project 75I, which will add 6 more submarines between 2025–2030. The submarines will be built by an Indian Shipyard in collaboration with a Foreign shipyard.
If everything goes as planned, the construction is expected to start in the early 2020s.
3–4 yet unnamed and larger “Arihant Follow on” Class Submarines
These will be Larger Nuclear Ballistic missile submarines carrying ICBMs like the K-5 (Which is under developement). Not much information is available on this, but all the 3–4 submarines can be expected by around 2030.
6 Nuclear Attack Submarines
The Govt. of India sanctioned this project in 2015, after which the design and development started. Again like India’s Nuclear Submarine programme , much information is not available, but construction can be expected to begin in the mid-late 2020s after design and development and all the 6 to be inducted by mid to late 2030s.
The 6 Project 75I and 6 Nuclear Attack Submarines will replace the 13 Shishumar and Sindhughosh Class Submarines in the Navy. So, say around 2037, 20 years down the line, the Indian Navy will posses around 26 Modern Submarines including 14 Nuclear Powered ones.