This seems a translation issue, here is the Sanskrit shloka and Hindi translation of Kalki Purana 4.15:

The religion of the Brahmanas is to render pure devotional service
unto Me. Being pleased by their devotional service, I incarnate, along
with Goddess Laksmi (Shri), from age to age.
Are there 10 avatars of Vishnu per kalpa?
There are infinite avatars of Vishnu, few of them are given in this post:
Mahavishnu's Avataaras?
10 or 24 are most famous among all avatars but they are not the only avatars.
The phrase संभवामि युगे युगे (sambhavami yuge yuge) is there in Shrimad Bhagavad Gita 4.8 too:
परित्राणाय साधूनां विनाशाय च दुष्कृताम्।
धर्मसंस्थापनार्थाय संभवामि युगे युगे।।4.8।।
And here is how different scholars translated it:
[Lord Shri Krishna to Arjuna]: Paritranaya, for the protection; sadhunam, of the pious, the followers of the virtuous path; vinasaya, for the destruction; duskrtam, of the evil-doers, of the sinful ones; and also dharmasamsthapanarthaya, for establishing virtue fully;-for that purpose, sambhavami, I manifest Myself; yuge yuge, in every age. -English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambirananda
[Lord Shri Krishna to Arjuna]: For the protection of the good and also for the destruction of the wicked, for the establishment of Dharma, am I born from age to age. -English Translation By Swami Adidevananda
[Lord Shri Krishna to Arjuna]: For the protection of the pious, the destruction of the evil-doers, and establishing virtue, I manifest Myself in every age. -English Translation By Swami Gambirananda
[Lord Shri Krishna to Arjuna]: For the protection of the good, for the destruction of the wicked and for the establishment of righteousness, I am born in every age. -English Translation By Swami Sivananda
[Lord Shri Krishna to Arjuna]: For the protection of the good people, and for the destruction of evil-doers, and for the purpose of firmly establishing righteousness, I take birth in every age. -English Translation By By Dr. S. Sankaranarayan
[Lord Shri Krishna to Arjuna]: To protect the righteous, to destroy the wicked and to establish the kingdom of God, I am reborn from age to age. -English Translation by Shri Purohit Swami