Everything about Gayathri Mantra

1) How many dieties are mentioned in Gayatri mantra and what is the best time to recite Gayatri mantra ?

There are 26 gods in Gayatri mantra 
tat - ganesh
sa - Narasimha
vi - vishnu
tu - siva
va - krishna
re - radha
Ni - lakshmi
yam - agni
bha - indra
Rga - saraswati
dE - durga
va - hanuman
sya - prithvi
dhee - surya
ma - Sriram
hi - seetha
dhi -  chandra
yo – yama
ya - brahma
na - varuna
cho - hayagreeva
da - hamsa
yaat – tulasi 

"uttamaa - taara kopitaa atridhaamata" -  best time for chanting Gayatri mantra is when stars are  still seen in sky before sunrise,
 "madhyamaa – soorya-sahitaa praataha sandhya" - medium time is when stars meet sun in the morning and in the evening.
 "adhama  lupta taarakaa"  - worst time is after stars disappear and only sun remains in sky

That’s why ancestors practice mantras  in  'braha muhurtam \ ' around 4am. 

"ashtottaram sahasram va, ashtottara satam tu, va, ashtaavimsati revaada gatarrim dasakam japet" 

One should chant 1008 times, or 108 time of 28 times or at least 10 times a day.

agnirmukham, brahmasirah vishnur hridayam, rudrassikha

agnirmukham,  - Gayatri has fire in face,
brahmasirah  - brahma the creator in forehead,
vishnur hridayam  - vishnu the protector in heart and
rudrassikha - Siva the destroyer on top of head.

So its a combination of all gods.

2. How many times  Gayatri Mantra should be chanted ?

3 times a day.

  a) 4 to 8 AM, 4 to 8 PM are Saattvik times.
  b) 8AM to 4PM is Raajsik.
  c) 8PM to 4AM is Taamasik.

The best time to chant Gayatri is around 6AM, 12 Noon and 6PM.  

Goddess Gayatri confers the boon of
Intelligence as Gayatri,
Protection as Savitri and
Learning as Saraswati.
Gayatri is therefore called Sarvadevata Swarupini.  

By chanting at least twice a day - morning and evening - the sin  committed unknowingly that day are destroyed and left over Karma is burnt. The person gets closer and closer to Divinity.Chant the Mantra 3 times when you are having a bath - it will serve the dual purpose of chanting as well as performing Abisheka to the Mantra.

Chant 3 times before eating - you would have not only purified the food, but also offered the food to the Mantra.
If you do it every day, you need not chant any other Mantra. It protects you from calamities and confers upon you intelligence


PS : Shamelessly but respectfully copied form Quora !