How many generators are in the world?

Answer:

The La Rance Tidal Power Plant has been in operation since 1966 and has a capacity of 240 MW.

South Korea currently (July, 2012) operates the largest tidal power plant in the world.

Jiangxia Tidal Power has been in operation since 1985, but only produces 3.2 MW.

SeaGen, in Northern Ireland, produces 1.2 MW.

In all, there are about 8 so far which produce more than 1 MW, and numerous smaller projects in operation.