Gfyuki

I came to know that a human breathes 21,600 times per day according to Yoga sastra.

Which shloka explicitly says this?

This is mentioned in Yogic texts as well as in numerous Tantras.

For example the Vigyana Bhairava Tantra says:

The breath is exhaled with the sound 'Ha' and inhaled again with the
sound 'Sa'. Thus the individual always repeats this particular mantra
Hamsa. (155)

This japa of the Devi which was previously indicated, (being repeated)
twenty-one thousand six hundred times during the day and night, is
easily available and difficult only for the ignorant. (156)

I am quoting from p 68 of the PDF.

A related and interesting article you can read is: Time and the Kalachakra.

Hamsa Upanishad, Yoga Upanishad which belongs to Shukla Yajurveda, mentions that.

अथ हंस ऋषिः ।
अव्यक्ता गायत्री छन्दः ।
परमहंसो देवता ।
अहमिति बीजम् ।

स इति शक्तिः । सोऽहमिति कीलकम् ।
षट् सङ्ख्यया
अहोरात्रयोरेकविंशतिसहस्राणि षट् शतान्यधिकानि भवन्ति ।

English Translation by K. Narayanasvami Aiyar:

(Here the performance of Ajapā Gāyaṭrī is given).

"Now Hamsa is the ṛshi; the metre is Avyakṭā Gāyaṭrī; Paramahamsa is
the ḍevaṭā (or presiding deity) 'Ham' is the bīja; 'Sa' is the śakṭī;
So’ham is the kīlaka. Thus there are six. There are 21, 600 Hamsas
(or breaths) in a day and night.