Is ablution valid if one forgets or doubts the number of times a limb has been washed, or if one lacks concentration during ablution?

If there is doubt about washing an obligatory limb during ablution (wudu'), it must be performed, and whatever follows it must be redone to maintain the proper sequence. However, if there is doubt about washing a recommended limb, it is recommended to perform it based on the principle of al-aqall al-mutayaqqan (the least that is certain).

If the doubt occurs after completing the ablution, it should be disregarded.

As for someone who experiences frequent doubts and obsessive thoughts (waswas), they should ignore them and not pay attention to them, whether they occur during ablution or after it.

Distraction during ablution does not affect its validity as long as the intention (niyyah) is present and has not been broken.