How much do we used keywords in our landing page or meta tags to get better rankings in SERPs?

There are no SET RULES.

You can use one particular key phrase per page or you can be savvy and utilize your organic text and similar or alternative terms as well as google is on a daily basis improving on understanding meanings and definitions of words.

Although your keywords ratio counts (your targeted kw over your total number of words per page) to some extent, it is not what it used to be 10 years ago. your actual authority, originality, and uniqueness of content will count many times as much as your keywords of choice.