Is the prayer of one who doubts the number of rak'ahs and relies on another valid, even if he does not perform the prostration of forgetfulness (sujood as-sahw)? And does doubt during prayer necessitate the prostration of forgetfulness, even if the doubt subsides?

If a latecomer (masbuq) doubts the number of rak'ahs, there is no harm in him relying on the prayer of the latecomer next to him, as this falls under acting upon the predominant assumption (ghalib al-dhann), as Sheikh Ibn Uthaymeen mentioned. It is permissible to build upon the predominant assumption or certainty, and he should perform the prostration of forgetfulness (sujud al-sahw) in both cases, unless the doubt disappears and he becomes certain of the correctness of his prayer, in which case the prostration of forgetfulness is not obligatory for him.