If Google Analytics tracks by dropping cookies what happens if a visitor has cookies disabled?
Simple answer - no tracking.
This is one of the reasons why comparison of GA data with absolute measures - such as ecommerce transactions - often will not match. Many content blockers will also block GA tracking, for example. Getting a good answer on how large number is can be hard to arrive at. On the sites I work on the % of users using adblockers is around 20%, how many of these block GA I do not know right now.
However, you can actually use GA to track whether your visitors are using ad-blockers. Google around and you'll find solutions for this. For example: https://www.kaushik.net/avinash/adblock-tracking-google-analytics-code-metrics-reports/
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