Assalaamu Alaykum. Can you perform salatul tawbah (prayer for repentance) for multiple sins or do you have to perform one salat for each sin. Also is it ok do tawbah when I'm in sujood in the salat or shall I do it in dua after the salat

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Tawbah (repentance) prayer is an act determined by the Sharee‘ah.

We have not come across any scholar who suggested that the sinner has to perform a Tawbah prayer for each sin he commits. or that he can perform one Tawbah prayer for more than one sin. Apparently, there is no religious impediment to that just like it is permissible for the sinner to seek Allah’s forgiveness for more than one sin he has committed.

Verily, deeds are judged by the intentions.

Scholars of Hadeeth commented on the Hadeeth on Tawbah prayer saying, “The Hadeeth reads, ‘…then, he should seek forgiveness’; this means to turn to Allah and repent from one's sin by declaring one's regret, giving up the sin, and resolving never to commit it again.”

This opinion was stated in Mirqaat Al-Mafaateeh by Al-Mulla Al-Qaari. If seeking forgiveness in this context refers to repentance, it is for all the sins one has committed and not for one sin. Therefore, you do not need to perform Tawbah prayer for each sin, but you can perform one Tawbah prayer for multiple sins and, hopefully, you will be forgiven.

As for the question regarding whether the person should seek forgiveness in prostration or after ending the Tawbah prayer, you are allowed to seek forgiveness in prostration since one is the closest to one's Lord when one prostates one's own self before Him. However, seeking forgiveness in Tawbah prayer should be after ending it as explained by the jurists. Al-Ghurar Al-Bahiyyah fi Sharh Al-Bahjah Al-Wardiyyah reads, “Tawbah prayer consists of two Rak‘ahs; when the person commits a sin, he should [perform Tawbah prayer and then] seek forgiveness of Allah after them [the two Rak‘ahs] as Al-Mahaamili underlined…”

Moreover, Haashiyat Al-Bujayirmi ‘ala Al-Khateeb reads, “The two-Rak‘ah prayer of Tawbah is performed when the person commits sins, even minor ones, according to the preponderant opinion. He should perform Tawbah prayer and seek forgiveness after them... Al-‘Anaani, stated: "The person should perform Tawbah prayer, then seek forgiveness. This is evidenced by the statement [of the Prophet mentioned in the Hadeeth on Tawbah prayer]: '...then, he should seek forgiveness.' Moreover, Tawbah prayer is a means to have one’s repentance accepted, and, verily, the means should be done before the objective."

Allah Knows best.