What if I forget how many rakat I prayed?

Build on what you are sure of

The Prophet ﷺ said: "If any of you is in doubt in his prayer and does not know how many he has prayed, three or four, let him throw away the doubt and build on what he is certain of, then make two prostrations before saying salam" (Sahih Muslim 571). He added that if it turns out you prayed five, the prostrations make it even; and if you prayed exactly four, they humiliate the Shaytan.

So the rule is simple: the lower number is the certain one. Unsure whether it's your second or third rakah of Dhuhr? Count it as the second, pray two more, then do the extra prostrations.

When you have a strong feeling

Sometimes you're not 50/50: you are fairly sure it was the third. In another narration the Prophet ﷺ said to aim for what is most likely correct and complete the prayer on that basis (Sahih al-Bukhari 401). That's why scholars differ a little:

• Shafi'i and Maliki: always take the lower number.
• Hanafi and Hanbali: if one number feels clearly more likely, go with it; if not, take the lower number.

Either way, finish with the prostration of forgetfulness. See What do Muslims say when they pray?

Doubts after the prayer

Once you've said salam, a doubt about how many rakat you prayed is ignored, according to scholars, unless you are actually certain you missed one. If you're certain, and it's soon after, stand up and pray the missing rakah, then do the prostrations. If a lot of time has passed, repeat the prayer.

If it happens all the time

Constant doubting can be waswasa, whispers from Shaytan meant to make prayer feel exhausting. Scholars advise people with frequent doubts to stop acting on them and assume the prayer was complete. Ignoring the whisper is how it fades. If anxiety around worship is affecting you a lot, talk to a trusted imam, and to a doctor or therapist if needed.

Most new Muslims lose count because they're still learning the words. That gets easier quickly. See How do I pray as a Muslim?