If my Forerunner dies and I am playing Rengar Pridestalker, do the tokens summoned from Forerunner give plus 1 to other units?

Yes. Each Mech token is "played" when Forerunner's Deathknell resolves, so Pridestalker triggers once per token — two triggers, each giving a unit +1 Might (not one +2).

You can put both +1s on the same unit or split them between two units. Since the tokens are played to your base, they're also legal targets for the bonus themselves if you want (the ability says "a unit," not "another unit").