It depends on how much time you have to dedicate to things. You can make a full time income from a single niche if you do it right.
When I first started I had one niche and focused everything on that. Once that started paying back I was able to outsource many tasks which saved me time and allowed me to venture into more niches through different sites at the same time.
The biggest thing here is don't try to run before you can walk. Find your ideal niche and make that pay before starting on more.

You can do several, but just starting out, while you're doing the training it is advisable to do one until you get the hang of it so you won't be overwhelmed. At least that's my advice!

Jeff

That depends on you. Some people are happy with one niche. I am in favour of spreading your income and work in maybe ten niches. If you have just started start with one