There is no official limit decided but generally people mention 100 to 255 characters for keywords. Different type of search engine sets their own rules. If anyone using maximum keywords, it could be considered overstuffing. So you can use less than of 10% of the total words of a single web page. It can be good practice for search engine optimization.
Example:
<title>Keyword 1 | Keyword 2 | Keyword 3 | Keyword 4</title>
<meta name="description" content="Most searchable area of remedy of any kind of disease. For Writing a blogs on different types of disease, srcraftblog.com could be best platform for bloggers." />
<meta name="keywords" content="names of diseases,types of disease,human diseases list,list of common diseases " />
