What is the ruling on doubting the number of times one has washed the limbs during ablution (wudu) and ritual bath (ghusl), especially with the recurring whisperings (waswas) concerning the intention (niyyah)? And what is the correct way to wash the neck and the folds of the skin during the ritual bath after impurity (ghusl from janabah)? And what is the optimal way to deal with these whisperings (waswas)?

First, you must repent to Allah for your sins, for watching forbidden acts, and for practicing the unlawful habit.

What you have mentioned indicates that you are afflicted with waswas (obsessive whispering), and its cure is to ignore it and pay no attention to it.

If Satan makes you imagine that you have not washed, then assume that you have washed.

For ghusl (ritual bath), it is sufficient to cover the entire body with water once, along with the intention.

Scholars have differed regarding the repetition of washing the entire body three times. Some held the view of limiting it to one time, while others considered repeating it three times. Whichever way you do it, it will suffice, as the aim is for the water to reach all parts of the body.

The neck is washed with the body after washing the head three times.