Well practically, a whole tube is an obvious limit as then you run out.
But really, a pea sized amount is usually more than enough. Remember though that toothpaste really isn’t designed for this, it often has particles in that are great for whitening teeth but annoying and scratchy down below. And you definitely shouldn’t be letting those get in your pussy.
So if you do use toothpaste you’re best off getting the gel only ones. But really either invest in some proper clit balm, or make your own by getting some menthol rub, the stuff for achey muscles, and mixing it with some lotion. Far more effective (start with TINY amounts and build up), lasts much longer and can get super painful if you’re a serious masochist.