Every single "bike" tail light I found to be woefully inadequate. They simply aren't bright enough or visible enough. Here is what I used and for just $9, it came with 2 lights so I originally planned to install both. What I found was that ONE was so bright a second simply wasn't necessary:

Amazon.com: LEMONBEST® 2 x New 17cm General 6W Super Bright COB Bulb Car LED RED Daytime Running Light DRL Driving Lamp black housing: Automotive

I have yet to find any "bike" light that even comes close to the brightness of these. I don't know how many LEDs there are, too many to count, but I'd say at least 50 LEDs are on each light. The only down side is that they need a 12V power source, so I just used a little Velcro strap along with a little Lithium rechargeable battery pack I found in order to power it:

Amazon.com: Turnigy 2200mAh 3S 20C Lipo Pack: Toys & Games

Works great I strongly recommend!