Permissibility

Surah Al-Hajj (22:77)

“O you who have believed, bow and prostrate and worship your Lord and do good – that you may succeed.”

يَـٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ ٱرْكَعُوا۟ وَٱسْجُدُوا۟ وَٱعْبُدُوا۟ رَبَّكُمْ

Surah Al-Alaq (96:19)

“Indeed, I am Allah. There is no deity except Me, so worship Me and establish prayer for My remembrance.”

إِنَّنِىٓ أَنَا ٱللَّهُ لَآ إِلَـٰهَ إِلَّآ أَنَا۠ فَٱعْبُدْنِى وَأَقِمِ ٱلصَّلَوٰةَ لِذِكْرِىٓ

Evidence for permissibility of counting rakats in Salah;

1.  Fatimah bint al-Husayn (رضي الله عنها) used to move her ring (from one finger to another) to count her rak'ahs in prayer.
   (Musannaf Ibn Abi Shaybah, Vol. 2, p. 488, Hadith 10706)
2. Shaykh al-Albani (رحمه الله) noted:
   "The Sahaba used pebbles, and Fatimah bint al-Husayn used her ring—so there is no harm in using beads if one fears losing count."
   *(Silsilat al-Ahadith al-Sahihah 5/280)*