31 from half the tribe of Manasseh, designated by name to come and make David king—18,000;

Read 1 Chronicles 12:31 Using Other Translations

And of the half tribe of Manasseh eighteen thousand, which were expressed by name, to come and make David king.
Of the half-tribe of Manasseh 18,000, who were expressly named to come and make David king.
From the half-tribe of Manasseh west of the Jordan, 18,000 men were designated by name to help David become king.