There are 22 kinds of doubts which one can have while praying. Out of these, 7 doubts are those which invalidate the prayers, and 6 are those which should be ignored. And the remaining 9 doubts are valid doubts.
Doubts Which Make Prayer Void
1174. The following doubts make prayers void:
Doubts about the number of Rak'ats occurring in obligatory prayers which consist of 2 Rak'ats, like, Fajr prayers, or prayers offered by a traveller. However, doubt about number of Rak'ats in Mustahab prayers or namaz of Ihteyat does not make the prayers void.
Doubts about the number of Rak'ats occurring in prayers consisting of 3 Rak'ats, that is, Maghrib prayers.
Doubt occurring in prayers of 4 Rak'ats as to whether one has performed one Rak'at or more.
Doubt in prayers of 4 Rak'ats before going to the second Sajdah, as to whether he has performed 2 Rak'ats or more.
Doubts between 2 and 5 Rak'ats or between 2 and more than 5 Rak'ats.
Doubts between 3 and 6 Rak'ats or between 3 and more than 6 Rak'ats.
Doubt between 4 and 6 Rak'ats or between 4 and more than 6 Rak'ats, with the details which will come later.
1174. If a person has one of those doubts which makes prayers void, it is better for him to break the prayers if the doubt persists. In fact, he should prolong thinking about it so that the form of namaz changes, or till he loses all hope to ascertain the situation.