What is the maximum number of keywords I can put in the meta keyword tag?
Most people recommend no more than 200 words.
Will putting a lot of words penalize me in any way?
Not unless you repeat words or phrases.
I have read that the keyword is not all that important, is this true?
Yes - in that Google no longer pays them any attention and concentrates solely on page/post content. Yahoo also announced recently that it too would longer be indexing sites via meta-keyword.
does the order of the words in the phrase matter
No - unless you're hoping to pick up on complete search phrases in some of the older engines.
do I have to list every phrase in every possible order
No! Do not repeat any word more than twice in the whole keyword list if you can possibly manage it.
Summary: Don't agonise over keywords. They're simply not worth the effort these days. Focus on providing good solid page/post content and use your keywords in page headings, subheadings, link text and within the first few sentences on each page.