Hi jhansen,
Welcome to 5 Star. It's a great question. My STRONG advice if you are a newbie or fairly new at this is to stick to one site. ESPECIALLY if it's working.
You could expand into other related areas or add pages about other related products but stay on the same theme. So if your site is about dog training, add other dog or animal training content and products for example. Here's a bunch of really good advice.
Martell James a HUGE, experienced, well known, super affiliate, guru and trainer who used to train affiliates to build a bunch of niche sites. Now with more experience he's changed his mind about that strategy. In answering the question "what would you do, knowing everything you know now, if you had to start over" James said
"If I HAD to Start Over - What would I do Different???" I would build one site. Just one. One.
This thread has more info too
Think Small to Earn BIG! Niche Marketing Insights