You can use more thana one niche on your website, however, before you starting building contents on multiple niches you need to consider a couple of things.
What is your website about? If your website is a company website, using different niche than the niche of your company can be detrimental. For instance, your website is for your graphic designing company, if you start publishing on parenting, your website would not look professional. However, if you create a blog section, you can publish on other topics.
If your website is an ecommerce site, having contents on different niche on the main domain might not look professional, however, you can have a blog, even a forum and post on other topics.
In case your website is a blog, you can publish on more than one niche. You can use different niches as your content categories.