What is the ruling on one who doubted the number of prostrations, and based [their action] on the lesser [number], and prostrated an additional prostration, then remembered their supplication in the first two prostrations during the Tashahhud, so they prostrated after the Tasleem for an additional prostration; and is the prostration of forgetfulness legislated for one who doubts the number of rak'ahs or prostrations then becomes certain there was no error?

If someone doubts having missed a prostration and bases their action on the minimum (i.e., assumes they missed it), then it becomes clear that they did not miss it, their prostration of forgetfulness (sujud (definition) al-sahw) is correct. However, if someone doubts the number of rak'ahs or prostrations and does not perform what they doubted having missed, then their doubt disappears, and it becomes clear to them that they did not miss anything, their prayer is valid, and they are not required to perform prostration of forgetfulness.