Question: I know that there is one place where we can actually maintain our keyword in Joomla Global Configuration-> Metadata Settings -> Site Meta Keywords, but is more like a place to dump all keyword of our website. Is it possible for us to maintain our keyword by each page? Will there be any effect on SEO?

Answer: 
Yes you can. Joomla! provides a mechanism that starts from the general to the particular, the metadata management is a clear example, let me explain you how this works:

* The small modular part here is the article. If you go to each one, inside you have this option in the tab called **"Publishing"**, this replace the data in the menu item.
* If you go to your menu items, inside you can see a tab called **"Metadata"**, from here you can replace the data in the **Global Configuration**.
* If you don't add anything for articles or menu items, then the data in Global Configuration is the default data.

There is a limit for those, this depends on the search engine. Use as a starting point something like:

* Meta Description: **max 160 characters**.
* Meta Keywords: **around 10 terms**.

And related to whether it affects, I think yes, but for good, because you'll have a more diverse amount of data.
Hope this helps!
Effect would be normal SEO effect for metadata on a web page, nothing special above that.

Every menu item has a metadata tab, and every content item has a place for metadata on the Publishing tab. You can use those for page-specific keys. IIRC, the menu item one overrides the content item one, but I'll leave testing that to you.