Subhabhivandanam !
According to the Vedas and Scriptures, there are mainly six kinds of incarnations of Lord Narayana.But in all, there are innumerable number of other avatars taken depending on need. This is also confirmed in Srimad-Bhagavatam (1.3.26) where it is said that there is no limit to the incarnations of the Supreme Lord, just as there is no limit to the waves of the ocean. Also, depending on the roles they play, some avatars can be overlapping to more than one kind...
These are six kinds of avatars:
1.Purusha-avatara --Karanodakasayi Vishnu, Garbhodakasayi Vishnu and Ksirodakasayi Vishnu
2. Lila-avatara ---Matsya, Varaha, Kurma, Narasimha, Vamana, Parasurama, Rama, Krishna, Bala-Rama, Kalki.
3.Guna-avatara-- Brahma (Rajas), Vishnu (Satvic) and Siva (Tamas).
4.Manvantara-avatara :there are innumerable manvantara avataras (504,000 Manus manifest in the life time of one Brahma): Main 14 also called as vaibhava-avatars: Yajna, Vibhu, Satyasena, Hari, Vaikuntha, Ajita, Vamana, Sarvabhauma, Rsabha, Visvaksena, Dharmasetu, Sudhama, Yogesvara and Brhadbhanu.
5 .Kalpa-avatara-- Kumaras, Narada, Varaha, Matsya, Yajna, Nara-narayana, Kardami Kapila, Dattatreya, Hayasirsa, Hamsa, Dhruvapriya, Rsabha, Prthu, Narasimha, Kurma, Dhanvantari, Mohini,Vamana, Parasurama, Raghavendra, Vyasa, Balarama, Krishna, Buddha and Kalki. (All these 25 avatars happen every one day of Brahma!)
6.Saktyavesa-avatara --Kapila, Rsabha, Ananta, Brahma, Catuhsana, Narada, Prthu, Parasurama, Buddha..
Nowhere in the scriptures Lord Venkateshwara is mentioned as lila or kalpavatara.Lord Venkateswara is also known as Kali yuga Pratyaksha Daivam.Lord Venkateshwara is not an avatar of Vishnu but Vishnu himself...he descended from vaikunta (I think you know all the reasons )
The last avatara among the Dasavatara is Lord Sri Kalki. It is also written that the twenty-fifth kalpa (in Bhagavata Purana) and final lila-avatara is Lord Kalki.
yada yada hi dharmasya glanir bhavati bharata
abhyutthanam adharmasya tadatmanam srijamy aham
paritranaya sadhunam vinasaya ca dushkritam
dharma-samsthapanarthaya sambhavami yuge yuge
(Bhagavad-gita 4.7-8)
“Whenever there is a decline of dharma (righteousness) and a prevalence of adharma (unrighteousness), the Supreme Lord appears, millennium after millennium, to deliver the pious and annihilate the dishonest, as well as to re-establish dharma.”
In Kali-yuga, when there is a terrible and extensive rise of irreligion leading to an ever-increasing defilement of society, the demigods will take shelter of Lord Vishnu. Understanding the desires of the demigods, the Lord will perform the pastime of appearing in the previously described village of Sambhala and will accept Vishnu Yasa and his wife Sumati as His parents. He will appear on the 12th day of the waxing moon in the month of Vaisakha (April-May). Reaching adulthood, He will annihilate all the mlecchas of the dwelling place of Buddhists, as well as Kuthodevi, the wife of the Rakshasas of Kalkanja, and all other mlecchas. He will even destroy Kali and re-establish dharma.
It has also been said that Lord Kalki will obtain the Vedic knowledge from Parasurama and the military science from Lord Siva. The horse on which He will ride while destroying the wicked will be white in colour.
http://www.bvml.org/SBBTM/kalki.html
http://www.vaikunt.org/AyyaVaikuntar/avatars.htm