Your question can't be answered unless you know your audience. This means you need the actual data from your users. Of course, I'm not just going to say that without giving you some type of estimate--but you should know this is highly inaccurate compared to actual statistics for your website.
If we want to guestimate, though... (cringe) then you have to think of a few different scenarios. If your website is a world-wide site, then you should know that 39 million people are completely blind in the world, and 285 million people are visually impaired in some way. If you are looking at just North America, then to my knowledge at least 1.3 million Americans are legally blind--which was roughly .3% of the population in 2010. Not all 1.3 Million people will be using your site; however, so if you want even more detailed statistics... well, you see where I am going.
When people think ADA they mostly think Blindness, but Deafness is just as important. There was a study in 2002 which stated approximately 1 million Americans were functionally deaf. This means subtitles, closed captions, etc would be very important for any videos or audio you might have on your site.
I would love to go on about Dyslexia, Color Blindness, etc but those are not necessarily covered by ADA Guidelines--they are basic UX practices--and your question doesn't ask about those... (Cry) (10%~ of people in the world are color "blind" (deficient), and as much as 15% of Americans have Dyslexia.)
If your website is a government-owned site, than being being 508 Compliant is actually a legal issue. If your website is not ADA Compliant, you could be sued.
Anyway, it could be safe to assume that on a very, very loose scale your site could perhaps have .05% - .2% of visually or auditory impaired users. (Note: As I mentioned before, this is a loose statistic I calculated guessing on the Asker's user base and the average of Americans with disabilities. Also, I'm bad at math.)
There are many more impairments that effect users, so other impairments could total from 5% - 20%. (But only half of these will likely be included in ADA Findings, the others are mere UX-Best-Practices.)
Questions to ask yourself
Will you make an ROI if you make your website more accessible? For instance, eCommerce sites will lose out on a small percentage of sales if disabled or impaired users cannot use their site.
Do you have a UX Designer / Front-End Developer / Web Designer, or are developers doing the front-end coding? Back-End Developers usually have a basic knowledge of HTML but advanced knowledge of HTML is required to be completely ADA Compliant--meaning you'll need a Front-End Developer. If Back-End Developers try to do the development, you'll spend more time and money developing for ADA.
My Professional Recommendations
If you're not sure you're ready to spend the extra time, money, effort and headaches to make everything in your site / software accessible, I would suggest starting small. Make sure all CTAs are ADA Compliant and can be reached via screen reader, users using the TAB key, etc. Anything very important to your site--like navigation--make that compliant.
You could then use A/B testing with the non-ADA compliant site with the new ADA-compliant site, and see if you get more traffic to pages, more clicks on the CTA, etc.
Also, crunch your user numbers. Google Analytics is really easy to set up and can help a lot with figuring out your users. For more advanced information, you can set up a survey on your site with optional information they can submit to you to help you make the site more accessible.
Added references. Also, someone mentioned the user is asking about WCAG and not ADA / Section 508 compliance. So, here's my answer to that.
WCAG, Section 508 & ADA--oh my!
Section 508 defines the minimum level of web standards which, by law, must be developed by any government agency using the web as a means of communication. WCAG actually came first, and these were a set of recommended guidelines which became official in 1999. Since then, these two sets of guides have worked in tandem to help businesses touch everyone--no matter their disability.
Yes, the user is asking about WCAG which is where the three stage guides come into play. However; as I mentioned above, those guides will not dictate how well the application actually works for an impaired user. The only way to tell this is by using the software yourself as if you have that impairment which you are testing.
I hope my answer touches on enough of all of these differently labeled but very similar guidelines, so that the Asker can make the best choice for his/her business.
3 http://www.cdc.gov/ncbddd/hearingloss/data.html (Note: I got the original estimate somewhere else, but found this reference to be more informative.)
4 http://www.colourblindawareness.org/ (Note: 8% of males and 1% of females. I added these then rounded up for buffer.)