Sigh why do people not grasp that when Luke is told he is the last its a lie.
They lied to him about his father, kept his sister a secret and sent him off to kill his father and murder the Emperor with a couple weeks worth of training.
Its not a big stretch that there could literally be thousands of Jedi running around and Yoda/ ObI Wan would still tell Luke he's the last one, because if he is captured, or fails or falls to the Darkside he can't cause a whole new Jedi murdering spree to happen.