Meta keywords used to be very important, but no longer. These days most search engines ignore them and most webmasters do too. However, some people still believe there is a benefit, so it may be worth putting them on your website’s pages. However you should not overdo the keyword metatag since ‘stuffing’ (the practice of repeating key phrases over and over again in a keywords metatag) is frowned upon by search engines. Moreover, since the tag is ingnored by most search spiders, it is also a waste of effort.
There was never an official limit on the number of meta keywords you could have but, a good rule of thumb is that since an average page is generally around 500 words long, more than 50 words could be considered overstuffing.
Personally I keep keywords below 255 characters in total because most databases have that as a default maximum size for certain fields so you may well find those engines that do look at the keyword tag only look at the first 255 characters in any case.
If you don’t bother with the keywords metatag, nothing bad will happen. The same cannot be said for the ‘description’ metatag. Although it has only a minor role in SEO algorithms, Google often uses this tag for the content of the description displayed in search results. This it gets seen together with your page ‘title’ tag which is very important and definitely should be created individually for each page on your website.
Consider the ‘description’ tag as an advert, enticing people to click on your page. This does not mean using lots of superlatives. It mean concise descriptive text which highlights the benefit to the visitor of the page in question.