Mishnah 11 in Maseches Makkos deals with the question of how we determine how many lashes - or how many sets of lashes - the condemned man is able to withstand.
The Mishnah states: "We assess the number of lashes a person can withstand only in a number that can be divided into three" - the assessment is always made in a number that is divisible by three.
For example:
If the doctors assessed that the condemned man cannot withstand all thirty-nine lashes, we do not give him thirty-nine, since that would cause his death.
If they assessed that he can withstand seventeen lashes, since seventeen is not divisible by three, we give him only fifteen, which is divisible by three. This is what the Mishnah means when it says that the assessment is made only in a number divisible into thirds.
When the assessment may be changed:
If they assessed that he can withstand forty - meaning thirty-nine, as explained - and they began lashing him, and in the middle the doctors retracted and said that they had erred and he cannot withstand them all, he is exempt from that point onward, and we do not wait until he recovers in order to complete the rest. However, as long as the lashing has not begun, they are permitted to change the assessment - to add to it or to subtract from it.
If he turns out to be stronger than assessed:
If they assessed that he can withstand eighteen - a number divisible by three - and after they gave him eighteen it became clear that he is stronger than they had thought, and he could in fact have withstood the entire amount, all forty or thirty-nine, he is exempt.
A transgression that involves two prohibitions:
If he committed a transgression that involves two different prohibitions, and he is liable to two sets of lashes:
If they assessed that he is able to withstand one full set of thirty-nine lashes, and in addition at least three lashes for the second set, we give him forty-two: thirty-nine for the first prohibition and three for the second, and once he has received them he is exempt.
But if he cannot withstand more than thirty-nine, and not even the minimum of forty-two, it turns out that he received nothing for the second prohibition: he is lashed for the first prohibition, he heals, and then he is lashed again for the second prohibition.
Only if the assessment can include at least a few lashes - a minimum of three - for the second prohibition do we say that he has received his punishment for the second prohibition as well.
In summary: We learned that the assessment is always made in a number of lashes divisible by three; that as long as the flogging has not yet begun the assessment can be changed, either raised or lowered, but once it has begun, if it turns out that he has not the strength to continue he is exempt, and we do not wait for him to recover; that if it emerged after the fact that he was stronger than the assessment had determined, he is exempt; and that in a transgression that involves two prohibitions, if the assessment included at least three lashes for the second prohibition, he has discharged his liability for both prohibitions, and if not, he is flogged, heals, and is then flogged again.