How many languages can/should someone learn at once? I've heard that you're only supposed to do one at a time; what have you heard? Does the degree of relationship between the multiple languages affect the number?

I would think, in order to do well, one at a time would be best.

Some might do one at a time but I'd say that generally its safe to learn more than one at a time. Most people won't even attempt it but I've been learning German and Latin side by side for two years now. I started German long before I started Latin so that likely plays some role in it.

Many people are afraid of 'mixing up' two languages in their head and I can guarentee you that, while this may happen from time to to or that you might forget a word in one language and only be able to think of it in the other, this is not a large problem.

I would suggest that you try to start another language while you're still learning another one and see how it feels. If you don't like it then stop. What you've learned won't be wasted, I find that learning even the basic grammar from the first few lessons of a language is helpful and interesting, especially if its an exotic language.

There was a time when I could learn up to 4 languages at a time (they had to be from different families), although I wasn't aiming for fluency. Now I can only learn 1 or possible 2 maximum. (I'm 38 now. That doesn't help)

I agree. "Better to have one sharp knife than many blunt ones."

You can become proficient in multiple languages at the same time, but for fluency, immersion is essential.

Well said.

On the contrary I think it's best to study two or more languages at once. The more experience you have the better, and you will be able to use the same learning methods for both even if they aren't related.

Of course there's an upper limit where you'd just be biting off more than you can chew, but I suppose this would vary for each person.

I also think learning two languages is refreshing for your mind, specially if they're distant to each other. Studying one excessively might prove too tyring.

How long per day (if you're studying every day) should you study for each language? Is it different if you're studying just one?