39 stripes

A slave code from Alabama (1852) reads:

#993. The patrol has power to punish slaves found under the circumstances recited in the preceding section, by stripes, not exceeding thirty-nine. 

(The "patrol" was the officer in charge of making rounds on the various plantations to catch slaves who were off their master's plantation without permission or who were simply loitering. Any white, male slaveowner below 60 years old were required to be available for patrol duty -- seems similar to jury duty today.)

Why 39? The number comes from the amount of lashes Paul received from the Jews, according to 2 Corinthians 11:24. Jewish executioners were not allowed to go above that amount of lashes according to the law of Moses (later codified into Roman law). 40 lashes was considered a death sentence.