How many passengers are allowed when a learner is driving?

You can potentially have as many people as can be fitted in the car. So if you're learning in a 5 seater, then you can have your driving instructor and 3 more passengers with you. However this is not recommended because it distracts and makes the learner more nervous.

This is why most driving instructors give a one-to-one driving lesson to help the learner concentrate on the manoeuvres. Just imagine, if you were trying to parallel park and your judgement is not quite correct and you have several people screaming at you telling you to be careful as you're likely to hit the car parked on either side. You'd hate the feeling and you'd be so focused on not making mistakes that you'll end up making even more silly mistakes.

In other countries, I've seen instructors having other learners at the back waiting for their turn but honestly, this is not recommended.

If you feel having those other passengers with you will not impair your learning, then by all means, go for it, it is legal after all.