Back in 2009 Google announced that meta keywords was not a ranking factor. Your main focus should be title tag and meta description.

Keep in mind that meta description is not that important to Google's ranking factor but it's very important when it comes to click-through rate. So make sure you write very compelling meta description to achieve max clicks from Google and other search engines.

Just one time you can use meta keywords in a single webpage.

Don't go with count. Just use one time in meta, if it is required to use it again than use similar word.

It should be only one time on web page. And its length should be upto 125-130 characters

Meta keyword is the main part of SEO when ever you use that keyword that's the limitation of that keywords so this part of is necessary of the added in html pages.

Meta keywords are not so important, pay more attention to title and eventually the description. Just 3-5 meta keywords are enough!

255 characters are enough..