Of Manasseh also there fell away some to David, when he came with the Philistines against Saul to battle: but they didn't help them; for the lords of the Philistines sent him away after consultation, saying, He will fall away to his master Saul to the jeopardy of our heads.
"Captains of thousands"  (1 Chronicles 12:20).  This evidently refers to positions these defectors to David had with the tribe of Manasseh; but there is no statement here that they brought their `thousands' with them to David's cause.  Nevertheless, the death of Saul that followed very quickly very likely afforded an opportunity for adding many thousands to David's army.