Generally they take eleven Upaniṣads primarily, but there are 108 Upaniṣads. Sometimes the explanation is given that "Why we have got 108 beads?" "Because there are 108 Upaniṣad."
So chanting of Hare Kṛṣṇa with 108 beads means that we are surpassing the study of 108 Upaniṣads. Another meaning is that there are 108 damsels who served Kṛṣṇa with all respect at Vṛndāvana. So that is also another explanation, 108. We are serving Kṛṣṇa's servitors, hundred and . . . chief damsels of . . . at Vṛndāvana.
So these 108 Upaniṣads are described, īśa kena kaṭha praśna muṇḍa māṇḍūkyo taittirī . . . you can count how many: Īśopaniṣad, Kenopaniṣad, Kaṭhopaniṣad, Praśnopaniṣad, Muṇḍakopaniṣad, Māṇḍūkyopaniṣad, Taittirīyopaniṣad. Then aitareyas ca chāndogya bṛhad-āraṇyakam tathā: then Aitareya Upaniṣad, Chāndogyopaniṣad, Bṛhad-āraṇyakopaniṣad, Brahmokaivalyopaniṣad, Jābālopaniṣad.
There are so many Upaniṣad, 108 Upaniṣad. Āruṇeyopaniṣad, Garbho, Nārāyaṇo, Haṁsa. This Haṁsadūta, the name, the Haṁsa Upaniṣad, it comes from Haṁsa. Garbho nārāyaṇo haṁso bindūro nārada sikaḥ: Sika-upaniṣad, Nārada Upaniṣad.
- maitrayaṇi kauṣītakī
- bṛhaj-jābāla-tāpanī-
- kālāgni-rudra-maitri-
- subālakṣari-mantrika
There are so many Upaniṣad; we cannot remember them. Out of them, nine Upaniṣad, just like Īśopaniṣad, Kenopaniṣad, Kaṭhopaniṣad, Muṇḍakopaniṣad, Māṇḍūkyopaniṣad, Aitareyopaniṣad, Taittirīya Upaniṣad, just like, these nine Upaniṣad are very much popular, and generally people read these nine Upaniṣad.
Now, so far Upaniṣads are concerned, here are some of the notes from different Upaniṣads given by Bhaktivinoda Ṭhākura. He's quoting just like the statement of Upaniṣad, pūrṇam adaḥ, pūrṇam adaḥ pūrṇam idaṁ pūrṇāt pūrṇam udacyate (Īśopaniṣad, Invocation). This is statement of one Upaniṣad, that the Supreme is pūrṇam, means complete.