Siddhartha is the name from his childhood (birth name), up to the moment of enlightenment. afterwards load Buddha was known as Shakyamuni Siddhārtha Gautama in Sanskrit or Shakyamuni Siddhattha Gotama in Pāli.

Meaning:

Shakyamuni [Shakya + muni] "Sage of the Shakyas"

Siddhārtha [siddha + artha] "Does Meanful"

The meaning of Gautama is unclear.

Note:

Shakyamuni Siddhārtha Gautama is the name of the buddha belongs to current era.

according to the "Buddhavamsa" text of Khuddaka Nikaya section included there in the Sutta Pitaka of the Pāli Canon of Theravada Buddhism, there were 27 Predecessor Buddhas.