...to add to what Highlandermom has said,(If I remember correctly)I think the MLB team(s) gets the draft number from the MLB Commissioner's office for the players they intend to draft. Every draftee will have a "draft number," the draftee probably won't know what that number is. A player can also be drafted without even knowing a team is interested in him and a player can have a draft number but not get drafted.
bbscout will correct me if I am wrong.