A person can perform one prayer or more as long as his ablution has not been nullified. If one’s ablution becomes void, he should renew it. If the person doubts whether or not he nullified his ablution, then his original state of being pure remains and his doubt does not have any effect.
Sheikh `Abdul `Aziz Ibn Baz, former Mufti of Saudi Arabia, stated the following: “If a person doubts whether or not he nullified his ablution, then his original state of being pure remains and his doubt does not have any effect. This is based on the Prophet’s statement, when a man asked him about sensations he experienced in his stomach while praying, “Do not leave [the prayer] until you hear a sound or you perceive a smell.” (Reported by Al-Bukhari and Muslim)
The Prophet (peace be upon him) clarified that the basic ruling is that of purity until one is certain that something has occurred to nullify it. As long as the person is simply doubtful, his state of purity is sound and confirmed. Therefore, he may continue to pray, circumambulate the Ka`ba, read the Qur’an from a mushaf and so forth. This is the original ruling, which springs from the mercy of Allah; the easiness of Islam. All praises are due to Allah.”