The more the merrier. I use 3 taillights, a very powerful Dinotte 140 coupled with a PB Superflash and a Radbot 1000. The Radbot is brighter than the Superflash but runs through batteries much faster. My Superflash will last for weeks without having to recharge its batteries, but the Radbot's batteries need recharging about every 10 days or so.