how many mac vs pc users are there? What is the percentage breakdown
between them and please provide a credible source for this data.
Thanks!

I think there is not an exatly user numbers and percentage, market is
changing, maybe on Apple Macintosh newspaper or inner files you can
find out the related information.

16%
http://www.macworld.com/news/2006/07/20/marketshare/index.php

http://www.macworld.com/news/2006/07/20/marketshare/index.php
Apple?s U.S. market jumped from 4.4 percent in 2005 to 4.8 percent in 2006 

According to google analytics the number of mac users for the site I
manage is: 1.51%

I guess your best bet would be to find some browser/os statistics from a big site.

For example w3c which is used for all sorts of things, especially
computer development publishes its own stats:
http://www.w3schools.com/browsers/browsers_stats.asp

If you scroll down the page, you'll see a section on OSs where MAC
gets ~3.7%. Anything that isn't MAC in the below stats is PC.

Windows XP is the most popular operating system. The windows family
counts for nearly 90%:
2006  	        WinXP  	W2000  	Win98  	WinNT  	W2003  	Linux  	Mac
Sepember 	75.6% 	9.2% 	1.4% 	0.3% 	2.0% 	3.5% 	3.8%
August 	        75.2% 	10.1% 	1.4% 	0.3% 	2.1% 	3.5% 	3.6%
July 	        74.3% 	10.1% 	1.5% 	0.3% 	2.0% 	3.4% 	3.6%
June 	        74.1% 	10.6% 	1.6% 	0.3% 	2.0% 	3.4% 	3.6%
May 	        74.2% 	10.7% 	1.6% 	0.2% 	2.0% 	3.4% 	3.6%
April 	        74.0% 	11.2% 	1.8% 	0.3% 	1.9% 	3.3% 	3.6%
March 	        72.9% 	11.9% 	2.0% 	0.3% 	1.8% 	3.4% 	3.5%
February 	73.3% 	12.3% 	2.1% 	0.3% 	1.8% 	3.4% 	3.6%
January 	72.3% 	13.1% 	2.4% 	0.3% 	1.7% 	3.3% 	3.5%

It also notes:
Statistics Are Often Misleading

You cannot - as a web developer - rely only on statistics. Statistics
can often be misleading.

Global averages may not always be relevant to your web site. Different
sites attract different audiences. Some web sites attract professional
developers using professional hardware, other sites attract hobbyists
using older low spec computers.

Also be aware that  many stats may have an incomplete or faulty
browser detection. It is quite common by many web stats report
programs, not to detect new browsers like Opera and Netscape 6 or 7
from the web log.

(The statistics above are extracted from W3Schools' log-files, but we
are also monitoring other sources around the Internet to assure the
quality of these figures)

As stated before the best way to get this type of information is by
using browser information however this is still flawed. Many Mac users
use the browser that comes with their machine - Safari which is
exclusive to the Mac platform and cannot be run on a non Macintosh
platform.

Link 1: http://marketshare.hitslink.com/report.aspx?qprid=0

This shows the Safari market share to be 3.53% as of September O6, you
can see the change over time here:

Link 2: http://marketshare.hitslink.com/report.aspx?qprid=3&qpcustom=Safari

However some Mac users may be using the Firefox browser which will not
be accounted for in the 3.53% figure. Also, a higher or lower ratio of
Mac users can be found depending on what site you go to. Sites such as
Digg.com and the TWiT podcast network are popular with Mac users so at
sites like that Mac users may take up possibly 40 - 50% of the hits.

Link 3: http://www.macobserver.com/article/2004/10/29.6.shtml
by Brad Gibson, 3:15 PM EDT, October 29th, 2004

Link 4: http://www.macrumors.com/pages/2006/06/20060601164133.shtml

"U.S. Mac Market Share
1Q 2005: 3.8%
4Q 2005 (AI): 3.5%
1Q 2006 (ZDNet): 3.5%
1Q 2006 (AI): 3.6%

Worldwide Mac Market Share
1Q 2005: 2.2%
1Q 2006 (ZDNet): 2.3%
1Q 2006 (AI): 2.0%"

We can safely assume that the Apple US market share is between 3.53%
and 4%. The global share by looking at the data I would say to be
between 2.0% and 3.0%.

You need to look at is the "US Market Share" and "Global Market Share"
as with the Macintosh computer the market penetration worldwide is
different to simply in the USA.

If you are into webdesign and you need this data for working out
target audience for a site. If your website is technology centric,
offers a podcast, offers a mac discussion area in a forum you will
attract a higher than average percentage of Mac users as these are
services and things that many Mac users use compared to the mean of PC
users do not.

"16%
http://www.macworld.com/news/2006/07/20/marketshare/index.php"

Uh, that's a 16% _increase_.  If you're going to answer a question
about statistics, it would help to understand the most basic concepts,
like the difference between an absolute measure and a relative change
in a measure.