The general rule of H1 tags is to only use one H1 tag per page or section. The proper use of title tags is always in debate. But people tend to forget that there are multiple correct uses, and it usually depends upon doctypes.
HTML4, XHTML, and HTML5
With XHTML4 and XHTML, you should only have one H1 tag per page. Since there are only divs in HTML4 and XHTML, there is only one way to define hierarchy and that is using title tag sequence.
With HTML5, there are separate sections which can all have their own H1 tag. For example, header could have an H1 tag, and aside could have an H1 tag, and article could have an H1 tag. This is because HTML5 uses the semantic elements to establish hierarchy, and the title tags to understand the content in those elements.
Check your website’s doctype, and use H1 tags accordingly.
What is the purpose of the H1 tag?
The purpose of the H1 tag is to inform search engines of the main topic. If you don’t use H1 tags properly, it may not have a big negative impact on your SEO, but it will make it harder for search engines to understand your content.