RTL-SDR Blog V4, which uses the RTL2832U ADC and R828D tuner, has the following sampling rate characteristics according to its datasheet

The RTL-SDR Blog V4, which uses the RTL2832U ADC and R828D tuner, has the following sampling rate characteristics according to its datasheet:

• Stable maximum sample rate: around 2.56 MS/s (mega-samples per second)
• Absolute maximum possible: up to 3.2 MS/s, but at that rate you’re likely to drop samples. (RTL-SDR)

In practice, most users find that setting a sample rate between about 2.4 MS/s and 2.56 MS/s gives a good balance of bandwidth and stability. (RTL-SDR)

So the maximum is 3.2 MS/s in theory, but 2.56 MS/s is the typically reliable top limit.

Let me know if you’d like details on achievable bandwidth (which is approximately equal to the sample rate) or how that relates to your antenna setup.