Google webmaster guidelines > Design and Content guidelines > Guideline two of nine in this category states...
"Offer a site map to your users with links that point to the important parts of your site. If the site map is larger than 100 or so links, you may want to break the site map into separate pages."
Definitions
site map - A page on your site that has links to the important parts of your website for ease of user navigation.
Examples and Explanations
Site maps should be a useful way for your visitors to more quickly and easily find the information they are seeking on your site.
Often site maps look like an outline. An effective site map typically has only text links and does not need to point to every page of your site but it can if you feel it is helpful for navigation.
If your web site is small it is easy and fitting to have your site map point to each page, but if your website has dozens or hundreds of pages, consider listing in your site map only the "important" pages for navigation.
Site maps are made for visitors, but they also serve the secondary function of giving search engine crawlers an easy way to find the pages of your site.
Sometimes a webmaster might make the mistake of putting each and every page on their site map just to "please" the search engine crawler.
It is always important to remember when you are making decisions about your site to make those decisions with your visitors in mind, not search engines.
Making a site map for your visitors
To illustrate I will use this website as an example. Our website is around forty pages, and here is what our site map page looks like...
Notice I do not link to every page on this website, but I point to
the important categories.
I made the decision not to include every page because if I had a link to every guideline (there are 31) it would just be a page full of links that would not clearly show the organization of my site and would not serve the purpose of a quick reference to help my visitors find what they want. If my visitors want a list of all the guidelines they can choose "Guidelines" which I described as " A list of the Google webmaster guidelines and links to our guideline detail pages".
See our actual sitemap
As you surf around the web, start making a point of looking at the site maps of the websites you visit to see how other are making their site maps.
To see the site map of Google.com go here
You may notice that Google.com is not following part of their own guideline - "If the site map is larger than 100 or so links, you may want to break the site map into separate pages."
This illustrates why Google calls these "guidelines" instead of "rules"
If having a clear and concise site map that is useful to your visitors requires you to have more than 100 links on a page (this is rare) then you may consider doing so.
Key concept
The site map of your website should link to the important parts of your site. It should make finding information on your site easier for your visitors.
There should be less than 100 links on your main site map.
How to determine if your website is following this guideline
Sitemap
If your website does not have a site map for visitors you are not following this guideline. Create one for your site to correct this.
Look at your site map and determine if it accurately "outlines" your site. Does it make sense to you? Ask your friends if it make sense to them. Look at other websites that have clear sitemaps that have been useful to you or make sense to you. This can provide guidance and ideas for your site map.
Amount of Links
Count the links on your site map, if there are over a hundred you are not following this guideline. To correct this, make additional pages for your sitemap. Ensure that no one page of your sitemap has more than 100 links on it. To quickly check how many links are on one of your pages use the useful link counter tool below (see Resouces)
Resources
From other sources:
Great online tool to quickly find out how many links are on a page:
Online tool for checking the amount of links on a webpage