Every page for itself
The most common mistake is that you think it’s all about the website. Although it’s easier to explain how SEO works by talking about the website in general and most people only expect to see their website on the first page for a particular keyword, they tend to not pay attention to which page is actually related to that result.
In many cases, one page of your website has good results for multiple keywords that are not even related, and this isn’t really a big problem... well, sort of.
If you sell a product and want to optimize for that particular product keyword, you wouldn’t want your users to see your contact page in their search results.
To understand it better - each page of your website should be optimized for multiple keywords that are related.