If you fill the tank to the top with water, you could easily keep eight newts in there.
However, you need to do a hell of a lot of reading and perfect their tank first, even for the ones you have now to live very long.
The good news is Chinese firebellies are simple to care for as long as their basic needs are met. All they really need to be happy is a clean, cold aquarium with lots of live plants to hang out and rest in. It doesn't matter what kind of plants, just the cheap, fast growing pond weed type stuff you find in petstores is fine, It should be either left floating or at least allowed to grow across the water surface so the newts can rest within easy reach of the surface.
The only land area they need is either a small cork float or island, they will probably not use it once they're settled, but it should be there if they want it.
As well as the links Aaron has posted there's years and years worth of info on here in both the help section and the advanced Firebelly and Sword tail newt topics, but if you are unsure of anything at all ask away and we'll help if we can.
If you do decide to get more newts I would strongly urge you not to buy any more from the pet store. Petstore newts are highly stressed and often sick after suffering in transit from China and again in petstores. You can find healthy, captive bred ones for sale on here cheaply. I've added an old link, but sadly, its still 100% relevant today.
http://www.caudata.org/people/JM/firebelly.html