* Definition is subjective: What constitutes a "muscle car" is debated. Some definitions focus strictly on large V8 engines and performance, others include styling and marketing. A car considered a muscle car by one person may not be by another.
* Variations and models: GM produced numerous variations within model lines (different engines, trim levels, etc.) across several brands (Chevy, Pontiac, Oldsmobile, Buick, etc.) over many years. Counting individual variants would be a massive undertaking.
* Time period: The muscle car era itself isn't precisely defined. Generally, it's placed in the late 1960s and early 1970s, but some argue for earlier or later models.
Instead of a single number, you can find extensive lists and databases online detailing GM's muscle car offerings by brand and model year. These provide a more comprehensive understanding than any single count.
