you can put 2-3 main keywords in meta tag. if your keyword is short then u can put 4-5 keywords.

which meta tag ? description or keyword meta tag ? AFAIK keyword meta tag has no importance.

This limit define below on the meta space. Specially define in words limit. 

The meta tag is not exactly word its based on characters.You can use around 80 characters.

Use minimum 60 words in meta tags.

we can use 3 to 4 it depends on the character size.

I suggest you to select different keywords for different pages. With this practice you can have unique content and your keyword will not be conflict with other pages.

Generally 2-3 keywords are used in meta tags for SEO and this way we can also help in improving search engine ranking.

Mostly we used 2 3 keywords in Meta Tag.

4 keywords will be best if your keyword are short

2 3 keywords are used in meta tags for SEO.
we can improve search engine optimization in page ranking.

Meta elements are the HTML or XHTML <meta ... > element used to provide structured metadata about a Web page. Multiple elements are often used on the same page: the element is the same, but its attributes are different. Meta elements can be used to specify page description, keywords and any other metadata not provided through the other head elements and attributes.
The meta element has two uses: either to emulate the use of the HTTP response header, or to embed additional metadata within the HTML document.
With HTML up to and including HTML 4.01 and XHTML, there were four valid attributes: content, http-equiv, name and scheme. Under HTML 5 there are now five valid attributes: charset having been added. http-equiv is used to emulate the HTTP header. name to embed metadata. The value of the statement, in either case, is contained in the content attribute, which is the only required attribute unless charset is given. charset is used to indicate the character set of the document, and is available in HTML5.

For you Best...
Use maximum 70 character with space in meta title tag.
Use maximum 160 character with space in meta description tag.
Use maximum 250 character with space in meta keyword tag. (optional)

You cannot specify the exact number of keywords you can place, it completely depends upon the size(character length) of your keyword / keywords that you use.

Quote from: Bob on November 28, 2012, 07:38:20 AM
You cannot specify the exact number of keywords you can place, it completely depends upon the size(character length) of your keyword / keywords that you use.

You are right BOB. Google counts character, not keywords. So, if possible your all potential keywords should be come in  first 70 characters in Meta title. 

Its depend on your keyword if you have short keyword you can add 5 keywords

Maximum we can use 5 keywords in meta tags

you can put some 2- 3 keywords in a single page.