It's nice to see someone asking questions before acquiring newts. In a 20 gallon tank you can have more than two newts - it will depend on the species and the way the tank is set up. Healthy adult aquatic newts only need a small land area, such as a pice of floating cork bark.
I would recommend that you spend some time looking through the Help section - check out all threads about Fire-bellied newts (you can use the search function, look for "orientalis", "CFBN", "firebellied"...). It will give you an idea about what you'd be getting yourself into if you purchased newts in a pet shop. Most newts will refuse to go in water, they will not eat, many will have sores and ultimately, many will die. these animals are caught in China, transported in terrible conditions and often almost tortured with the "care" they receive in pet shops. Many pet shops will happily lie about the origin of the animals but beware, no one has ever heard of breeders supplying pet shops.
I would recommend getting captive bred newts. It might take a little while and you might have to raise them from a juvenile stage, but you will have healthy animals. And you will not be supporting the trade with wild animals. There are many species that are suitable, Chinese firebellies (Hypselotriton (Cynops) orientalis) are not the only ones. Other good beginner species are Spanish ribbed newt (Pleurodeles waltl), Japanes Fire-Bellied Newt (Cynops pyrrhogaster), Chenggong Fire-Bellied Newt (Cynops chenggongensis), Blue-tailed Fire-Bellied Newt (Cynops cyanurus), Japanese Sword-tail Newt (Cynops ensicauda). You can find care sheets
here. I would also recommend reading all you can
here, it will answer many questions.
