Around 358 Trident C-4 and D-5 missiles have be fired in both R&D work and operational tests up to the end of 2015. Total number of failures was 12 (7 of which were C-4's). The 5 D-5 failures were all in the R&D and early operational testing phases (like the first SSBN launch in the video below) and up to the end of 2015, 184 D-5 missiles had been fired, including all of the R&D firings at the start of the program, plus missiles used for ABM target use.
The 161 launches quoted are operational systems tests from SSBN's and this is most likely the first confirmed failure. This gives a failure rate of 0.6% which isn't bad for any guided weapon system
All listed here
Trident