It says six, but only five are listed!
'Rashi', Radak (1): The lottery chose Shim'i to be 10th (17); he was not listed here. Bereishis Rabah (94:9) says that when David made the Mishmaros, Yedusun had five sons, and his wife was pregnant with Shim'i. David saw with Ru'ach ha'Kodesh that the child will be a Rosh Mishmar, so he said six 1 . Rashi - based on this, a Piyut says "Rashum mi'Beten Lehipaked b'Mishmeres ha'Kodesh."
Malbim: Shim'i (17) was not listed here, for he was not yet 30 years old. His father served and took part in the lottery on behalf of him, until he reached 30.
Radak: Or, we say that Shim'i was a minor at the time, and not yet proper to sing and be a Rosh Mishmar until he matures. He is included in the number, for they taught him, and in the future he would be Rosh Mishmar.
What do we learn from "ha'Niba Al Hodos v'Hallel la'Shem"?
'Rashi': They played on the harp songs of praise, and he would prophesize.
