There are 11 of the Greater Tears of Life left, but there are thousands of lesser versions out there (the Narsi made many little ones before creating the Tears).
It takes approximately 10 lbs of the Lesser type, or 1 or 2 of the greater ones to perform the ritual -- which is performed by the Rhona (not the PCs).
The next ritual is due to be performed on a boat out in the Camal Sea. The idea being to perform it in a location where it will heal most of the Camal Sea, which will in turn return many of the rivers to life, and spread healing outwards.
In GM terms, doing the ritual out on the Camal Sea will heal the land enough to stop it from slowly reverting back to the Devastation (which is what is currently happening). That gives the refugees more time overall....