Most modern forms of Buddhism offer ordination as a renunciant, however, the predominate form of monastic Buddhism is the Theravada school(s) of Southeast Asia.
Suggest Reading
Bareau, André. The Buddhist Schools of the Small Vehicle. Translated from the French by Sara Boin-Webb. University of Hawai’i Press, 2013
Prebish, Charles S. Buddhist Monastic Discipline: The Sanskrit Pratimoksa Sutras of the Mahasamghikas and Mulasarvastivadins. Motilal Banarsidass, 2002.