Is it permissible to follow the congregants in the number of remaining rak'ahs when one forgets the number of rak'ahs already performed?

If you doubt the number of rak'ahs, build upon certainty, so rely on the lesser number, or build upon what you predominantly believe, even by looking at those next to you. Then perform the prostration of forgetfulness (sujud al-sahw) in both cases.