How many total <meta> tag available in X/HTML and Which should be used and which not?
see this tool http://www.ulhas.net/tools/tu-metagen
is there any pros to use all other meta tags other than meta description?
note: Google doesn't give any importance to meta keywords.
First of all meta description and then language and robots.
You may also need verify-v1 temporarily when you are validating against http://www.google.com/webmasters
Edit: The robots tag is a SEO "tool" which can let you fine tune how the robots operate on your website. Language allows you to state the language in which the page is written in.
Which tags are useful and which not?
All mega tags are useful, it is just that you use them when you need them. Also the link you provided does not say anything about usefulness or drawbacks or meta tags.
Use only those meta tags that you need, don't add not needed tags to your pages. This also makes page messy and longer to load.
Thanks :)
In terms of SEO, the description meta tag is what most search engines will usually show in search results, so that's quite important.
Google have said that they don't use the keywords tag at all. And the other searches engines value it very little, if at all.
The robots tag can be useful if you want to stop SEs indexing certain pages, but using robots.txt is the better option. Certainly, don't bother with "positive" instructions like INDEX, FOLLOW because that's what SEs do by default anyway.
The language and content type tags are best replaced with HTTP headers if you have that option. Same goes for caching, revisit-after etc but I'm not sure any browsers take note of that anyway.
Don't bother with the meta tags that relate to "creation" (i.e. author, copyright, generator and so on) unless you have a specific use for them.