The Mars Sample Return (MSR) mission of NASA and ESA (European Space Agency) involves the collection and return to Earth circa 2040 of samples from the surface and atmosphere of Mars. A “Tiger Team” of scientists, of which I was a member, released a report in 2024 on planetary protection from potential biohazards in the returned samples. This lecture develops the Bayesian statistical design used to determine how many subsamples will be needed for biohazard testing. There are no data available since the samples are still inside sample tubes on Mars. However, there is prior astrobiological knowledge, and a Bayesian statistical approach can direct how that knowledge can be used. The number of subsamples is seen to depend on a prespecified risk-tolerance threshold (e.g., one in a million) and three other quantities that originate from the Bayesian design. Monte Carlo simulation is used in a digital twin to compute the number of subsamples needed for biohazard testing of each sample tube returned from Mars.