Q. Is Buddha the 9th incarnation of Lord Vishu? If so Balarama is not the incarnation of Vishnu? Please tell me the quotes from Bhagavatham / Vishnu purana where it describes that Buddha is the 9th incarnation.
A. Srimad Bhagwatam mentions the following about the incarnations.
vrishnishu prapya janmani
rama-krishnau iti bhuvo
bhagavan aharad bharam
In the nineteenth and twentieth incarnations, the Lord advented Himself as Lord Balarama and Lord Krishna in the family of Vrishni [the Yadu dynasty], and by so doing He removed the burden of the world.
tatah kalau sampravritte
Then, in the beginning of Kali-yuga, the Lord will appear as Lord Buddha, the son of Anjana, in the province of Gaya, just for the purpose of deluding those who are envious of the faithful theist.
The incaranations of the Lord are unlimited and come like waves of river. There is no point in counting the number, the sequence is given as mentioned in Srimad Bhagwatam