My Schengen tourist visa is valid from 12/08/2015 to 12/08 2017, i.e.for 2 years as type C visa. I travelled from Mumbai on 26/08/2015, stayed in Amsterdam and came back to Mumbai on 20/09/2015 (24 days in schengen area). Now again I want to travel to Amsterdam in April 2016, which is beyond 180 days from the date of my first entry in Amsterdam (26/08/2015).
Do I need to apply for visa again?
You Schengen visa is valid until its expiry date. It can be used for more than one visit to the Schengen area if and only if it is not a single entry visa. Schengen visas can be single, two, or multiple entry.
In your case, if you have a multiple entry 2-year type C visa, you can use it until it expires, provided you leave the Schengen area within 23:59:59 of the last day of validity. Clearly, all of your visits to the Schengen area are subject to the 90-in-180 days rule:
"90 days in any 180-day period [...]". "The date of entry shall be considered as the first day of stay on the territory of the Member States and the date of exit shall be considered as the last day of stay on the territory of the Member States. Periods of stay authorised under a residence permit or a long-stay visa shall not be taken into account in the calculation of the duration of stay on the territory of the Member States."
