Assalaamu alaky kum,
Are the dhikr after the prayer, such as "astaghfirullah, astaghfirullah, astaghfirullah" and "allahumma antas salaam wa minkasalaam" only to be said after the fard prayers, or can they also be said after the sunnah prayers?
Jazakallahu khairyan

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.

The recommended Thikr (mention of Allah) which is recited after the conclusion of a prayer is specific to obligatory prayers, according to the view of most scholars.

Explaining the phrase "at the end of every prayer" from the Hadeeth of the Prophet regarding Tasbeeh (saying 'Subhaan-Allaah') Tahmeed (saying 'Al-Hamdu lillaah') and Takbeer (saying 'Allahu Akbar'), Al-Haafith said in Al-Fat'h: "What is apparent from his words is that 'every prayer' includes both obligatory and recommended prayer, but most scholars have interpreted it to be specific to obligatory prayers." [End of quote]

What indicates that such Thikr is specific to obligatory prayers is the fact that the Prophet used to say them upon his completion of obligatory prayers. Muhammad ibn 'Ismaa'eel Al-San'aani mentioned in At-Tanweer Sharh Al-Jaame' As-Sagheer, when commenting on the Hadeeth: "Whenever the Messenger of Allah concluded his prayer, he would say Astaghfirullaah (I seek forgiveness of Allah) 3 times; and then he would say: 'Allaahumma anta-ssalaam wa minkas-salaam, tabaarakta yaa dhal-jalaali wal-ikraam (O Allah, You are the One Who is free from all imperfections and from You is all peace, blessed are You, O Possessor of majesty and honour).' It is apparent that the phrase "when he concluded his prayer" is referring to obligatory prayers because they are the primary prayers."

Allah Knows best.