I read that two huge black holes collided or merged and so it created gravitational waves billion years ago , So now we are able to detect it & detected it. So what is the speed of gravitational wave ? Is it faster than light ? and are there many more to be detected ?
Gravitational waves travel at the speed of light.
There are many possible sources of gravitational waves. The two confirmed detections so far (14 Sep 2016) are merging black hole binary systems, but sources of (detectable) gravitational waves may include merging neutron stars, short period binaries containing neutron stars or white dwarfs, gamma ray bursters or supernovae.
For a brief introduction see Gravitational Wave Sources at the ALIGO site.