Overview
All kings in the current Chakri dynasty of Thailand are often referred to as King Rama in the English speaking world. The name Rama was adopted from the name of the Hindu God Rama, an avatar of Vishnu.
The use of the name "King Rama 'n'th" is in line with Thai practice of giving numbers to the king in the current dynasty. However, the translation was not exact and can give rise to some confusion as to whether this was actually the name adopted by the king on his coronation.
History
While "Rama" was used as a title for all the kings, it was not always taken on as the name. In the present dynasty, the first king to call himself Rama was Phra Mongkutklao or King Vajiravudh, who was the sixth to reign. His reigning title was Phra Mongkutklao Chaoyuhua (พระมงกุฎเกล้าเจ้าอยู่หัว); later in his reign, he preferred to style himself as Phra Ram thi Hok (พระรามที่หก, lit. Rama VI). It was presumed that he was influenced by the European practice of numbering the rulers with similar names while he studied in England.
Quite conveniently, it coincided with another practice of the Thai people. Traditionally, the name of the king is sacred and would not normally be said. Instead, people would refer to the king by other words, currently Nai Luang (ในหลวง) or Phra Chao Yu Hua (พระเจ้าอยู่หัว). When King Phutthayotfa Chulalok founded the dynasty, he was commonly referred to as Phan Din Ton (แผ่นดินต้น, lit. 'the First Reign'); and when his son inherited the throne, he was referred to as Phan Din Klang (แผ่นดินกลาง, lit. 'the Middle Reign'). That then became awkward when Prince Jessadabodindra (King Nangklao) became the third king, as the obvious referral would then be Phan Din Plai (แผ่นดินปลาย, lit. 'the Last Reign'), which did not sound very auspicious.
From Wikipedia (CC BY-SA 4.0).