The more the better. You'd really need a statistically significant number of labs repeating it to be sure. If the underlying study is scientifically sound then a single repeat is good, two repeats is better. 20 repeats is even better. Good luck getting 1 or 2, unless the study is an important work that introduces a useful new technique or is the basis for additional studies. Also good luck knowing whether 2 labs failed for every 1 published. Science is hard. Reporting needs to be improved and there should be grants available to fund repeat studies.