There are two main sizes of threads featured on the telescope side of T-Rings. First are T-Threads, which are 42mm in diameter, and second is M48, which is 48mm wide. You'll need to find out which size threads the telescope/accessory you're trying to attach your camera to has before deciding on a T-Ring. This is usually listed in the specifications or manual.
Technically, T-threads have M42 x 0.75 threads and pitch, whereas M42 threads are M42 x 1. However, many manufacturers often interchange the two labels, and it's generally safe to assume that T-Rings that say M42 will thread into T-threads. This size T-Ring is most common, and can accommodate up to APS-C size DSLR and Mirrorless cameras.
M48, on the other hand, is a slightly wider thread, hence why you may see the term Wide T-Ring. M48 is quickly becoming the more popular choice for thread size on telescopes and accessories as it can accommodate up to Full Frame size camera sensors without any vignetting like T-threads/M42.