These Yoginis are actually Bhagavati's mates or sakhi or sahachari. They are also called the Avarana Devatas . That is, they cover (or veil) the main Goddess, that is Bhagavati, in several layers ( avarana means layer).
Gods always like to be veiled and here these Yoginis provide that veil for Goddess Bhagavati. They also accompany serve Devi Bhagavati constantly. This also shows that it is exceedingly difficult reaching the ultimate destination or Bhagavati. There are several thousand crores of such layers (or obstacles) to cross before reaching her.
The number or type of such Yoginis is actually more than crores. But the principle Yoginis are 64 in number as said by you.
They are together called the Chatuh Shashti Yoginis. The Brihannandikeswara Purana (an upapurana) lists the following 64 Yoginis :
NArAyani, Gauri, ShAkambhari, BhimA, RaktadantikA, BhrAmari, PArvati,
DurgA, KatyAyini, MahAdevi, Chandraghanta, MahAvidyA, MahAtapA,
SAvitri, BrahmavAdini, BhadrakAli, VisAlakshi, RudrAni,
KrishnapingalA, AgnijwAlA, Raudramukhi, KAlarAtri, Tapaswini,
MeghaswanA, SahasrAkshi, VishnumayA, Jalodari, Mahodari, Muktakeshi,
GorarupA, MahAbala, Sruti, Smriti, Dhriti, Tushti, Pushti, MedhA,
VidyA, Lakshmi, Saraswati, AparnA, AmbikA, Yogini, DAkini, SAkini,
HArini, HAkini, LAkini, Tridaseswari, MahAshasti, Sarvamangala, LajjA,
Kaushiki, BrahmAni, MAheswari, KoumAri, Vaishnavi, Aindri, NArasimhi,
VarAhi, ChAmunda, Shivaduti, VishnumAyA and MAtrikA.
An explanation or nirukta of the word Yogini is given in KulArnava Tantram as follows:
YonimudrAnusandhAnAth GirijApAdasevanAth |
NirlinoPAdhivibhavAd YoginityAbhdhiyate ||
Meaning
She is known as Yogini, as she practices yonimudra, as she serves
the lotus feet of Girija (or Bhagavati) and also as she has
become devoid of upadhi.
KulArnava Tantra, Chapter 17, Verse 31.
Upadhi here means the attributes. For example, richness is the upadhi for the rich. In Tantra, Jiva's Jivatva is its upadhi. But it is ultimately the eternal Shiva which it does not know because of Avidya or ignorance. So, this upadhi is not eternal too. Once self knowledge is obtained the upadhi dissolves and the Jiva attains Shivatva.
The verse says that a Yogini's upadhi is also have been already dissolved. They know their true nature which is not different from Bhagavati Parameswari's. But they constantly serve her lotus feet too. And, Yonimudra is a secret mudra used in Devi's ritualistic Puja.
Also, that there are actually crores and crores of such avarana devatas or Yoginis is seen from the following verse which provides an explanation for the word Shakti:
Satakoti MaHadivyayogini PritikAranAth | TivrasphurtipradAnnAncha Shaktityabhidhiyate ||
Meaning
She is known as Shakti, as she is the reason for bliss (or happiness,
actually priti is happiness) for the hundred crores of great and
divine yoginis, and as she gives intense energy.
KulArnava Tantram, Chapter 17, Verse 32.
So, Yoginis are not exactly Devi's incarnations ( i am mentioning this specifically since you added the avatara tag to your question) but her mates.
You must be knowing about Goddess Lalita being constantly served by several devatas including Lakshmi, Saraswati etc. Goddess Lalita who resides along with Kameswara in the inner most triangle (in the bindu to be precise) of the Sri Chakra is similarly being constantly served by crores and crores of other such Yogini-Devatas.
