What is the ruling on one who swore by Allah several times to abandon masturbation, then forgot the number of times, and then wished to repent?

If one swears an oath multiple times concerning a single matter, only one expiation (kaffarah) is required, and this is the opinion of the majority of scholars. Ibn Abi Zayd al-Maliki mentioned that whoever repeats an oath regarding one thing is only bound by one expiation. Al-Nafrawi narrated from the Malikis that expiation does not multiply, even if the intention was to establish new oaths or if no specific intention was made, and whether the oaths were in one sitting or in separate sittings.

Similarly, Al-Hattab narrated from Ibn Rushd that expiation does not multiply according to Malik and his companions.

Sahnoon narrated from Ata' that if a man swears ten oaths and then breaks them, if it was concerning one matter, his expiation is one.

Al-Buhuti Al-Hanbali said: Whoever is bound by oaths whose legal implication is one, and he breaks all of them before performing expiation, then his expiation is one.

Therefore, the questioner owes one expiation for all the oaths he swore, unless he had previously offered expiation for some of them. If he had offered expiation and then swore new oaths afterwards, an expiation for the new oaths would be required.