Although vaiṣṇavism is ultimately a matter of one's inner mood and disposition towards Bhagavān, there are still some external symbols that vaiṣṇavas are supposed to display on their bodies.
One such important external symbol is the tulasī-mālā. There is a specific injunction in the Hari-bhakti-vilāsa for wearing a Tulasī-mālā. This injunction is as follows:
tataḥ kṛṣṇārpitā mālā
dhārayet tulasī-dalaiḥ
padmākṣais tulasī-kāṣṭhaiḥ
phalair dhātryāś ca nirmitāḥ
Translation: An individual must wear mālās offered to Krishna. Such mālās can be made out of tulasī leaves, lotus seeds, tulasī wood or the āmalakī fruits (Indian gooseberry). — (Hari-bhakti-vilāsa 4.307)
We should note that this verse is a vidhi-vākya (injunction). The term "dhārayet" (must wear) in the verse specifies the injunction. Grammatically, the term "dhārayet" is the causative imperative (ṇij-anta vidhi-liṅ) verbal form of the root √dhṛñ.
Thus, this verse is specifying an injunction to be followed by all vaiṣṇavas. To not follow an injunction given in the vaiṣṇava-śāstras can potentially lead to pratyavāya i.e. negative results (according to Caitanya-caritāmṛta, Ādi, 4.35). Therefore, all vaiṣṇavas are expected to wear mālās.
A doubt arises — No specific number of rounds have been specified in this injunctive verse. How many rounds should a devotee wear around their neck?
The answer is hidden in the verse itself. The above-mentioned verse is a vidhi-vākya (injunction), and the term "mālā" (mālāḥ) in the verse is in plural case. Although at first sight, the term "mālā" appears to be singular in case, but the plural case is clearly revealed in the final adjective "nirmitāḥ" at the end of the verse. The commentary on this verse by Śrī Sanātana Gosvāmī also specifies the plural term "mālāḥ". Thus, the verse is specifying that one is expected to wear "multiple mālās".
The exact number is still not specified. This is a unique situation where:
(i) we have an injunction.
(ii) a plural term is used in the injunction.
AND
(iii) an exact number is not ascribed to the plural term.
For such situations, there is a specific nyāya (maxim) mentioned in the Mīmāṁsā-śāstras. This is the maxim named Kapiñjala-adhikaraṇa-nyāya (maxim pertaining to the number of partridge birds).
In the Vedas, there is a statement found — vasantāya kapiñjalān ālabhet — "Offer multiple Kapiñjala birds in the spring sacrifice". This statement too is an injunction that has a plural case in the term "kapiñjalān". The exact number of Kapiñjala birds is not specified. However, according to this maxim, the number is fixed as 3 (three).
Similarly, in the Hari-bhakti-vilāsa injunction on wearing mālās, the exact number of mālās has not been specified, but a plural term "mālāḥ" has been used. Thus, according to Kapiñjala-adhikaraṇa-nyāya, the number is set to three. In this way, a devotee is expected to wear three mālās. The three rounds that we see nowadays are nothing but another way of wearing three mālās. Devotees sometimes wear more than three rounds to get extra results — adhikasya adhikaṁ phalam — "Greater efforts lead to greater results". (Sanskrit Proverb)
We see also in the verse that mālās can be made out of many things viz. tulasī leaves, Indian gooseberry etc. A question naturally arises — Why then do almost all vaiṣṇavas prefer mālās made out of tulasī wood?
The answer is that in the next few verses of the Hari-bhakti-vilāsa, special emphasis has been laid on mālās made out of tulasī wood. The verses mention tulasī wood multiple times — tulasī-kāṣṭha-sambhavām (Hari-bhakti-vilāsa 4.309, 310), tulasī-kāṣṭha-sambhūte (Hari-bhakti-vilāsa 4.312) etc.
A vaiṣṇava should not hesitate to wear tulasī-mālā at any point of time. The Hari-bhakti-vilāsa (4.318) clearly says — na jahyāt tulasī-mālām — "Never give up a tulasī-mālā". A vaiṣṇava should not think that they're ineligible to wear a tulasī-mālā because they're unclean or because they sometimes indulge in activities that are unwholesome. Bhagavān himself says:
tulasī-kāṣṭha-mālāṁ ca
kaṇṭhasthāṁ vahate tu yaḥ
apy aśauco 'py anācāro
mām evaiti na saṁśayaḥ
Translation: Persons who wear a tulasī-mālā around their neck attain me without a doubt, even if they're most unclean or engaged in forbidden acts. — (Hari-bhakti-vilāsa 4.322)
The tulasī-mālā is best the ornament of the vaiṣṇavas. It does not need further ornamentation by adding diamonds, pearls etc. In fact, pearls and diamonds have negligible spiritual value in front of tulasī. Devotees who wish to wear other jewellery can certainly make separate mālās, but the three-round tulasī-mālā is supposed to be like a queen ruling over the kingdom of the vaiṣṇava's neck. She should be given her distinct and unique position. 
— Article and Translations by Hari Pārṣada Dāsa. 11 June 2022 (Nirjalā Ekādaśī).
PS: The maxim on Kapiñjalas is also sometimes applied in statements that are not injunctions. For an example, one can see either the BBT Purport or Śrīla Viśvanātha Cakravartī Ṭhākura's commentary on Śrīmad-bhāgavatam (10.90.42). The devotee in the photograph is Garib Das ji, popularly known as "Tulasī Bābā" and he stays in Govardhana.
