The large majority of search engines do not use Meta Tags as part of their ranking algorithym. The most notable exception is Yahoo which does use them (at least the meta keyword) but it's weight in their algorithym is extremely low and essentially has no influence on a high ranking (it may be useful to jump up you from the 1,000th position or so). Some will claim Google uses Meta tags in its algorithym. This is entirely untrue. Google, however, will use a meta description tag if it is unable to discern a description for a webpage on its own (if the page has no text and no description in the open directory [dmoz] the it is likely Google will use the meta description tag in its SERPs). Please note that it is only using this description in its SERPs, not its algorithym.
Should you use Meta Tags in your site? Yes. They do have some affect in some search engines and even though that effect is almost zero it is still more then zero so is worth the time.
How much time should I spend on my Meta Tags? Ten minutes. Write a nice concise description of your page and throw in a sampling of keywords (which you should have them handy if you've optimized your pages properly). You should spend no more time then this on them. Use your time to promote your site and get quality inbound links.
How many keywords should I use? As many as you want. If you start to think you may have too many, you probably do. This means you need to divide your page into subpages with each one taking its own topic.