If it's your first season at the school and you sign less than 6, the assistant coach will sign player(s) to fill up to 6 for that class. This happened to me at Piedmont D3, I signed 5, figuring 2 walkons wouldn't kill me and was very surprised when recruiting ended and there was a kid from Alaska on the team that showed as recruited by me. I submitted a ticket and support said he was signed by the assistant coach. I did wind up cutting him after another season, but he wasn't bad, not walk on level bad.
Just to reinforce since it came up recently, you only get money for 6 scholarships. Also, you can only have 6 in one class. So you'll have money for 6 (plus postseason cash from the conference) and have to fill 11, which you can use transfers or JUCOs to sign more than 6 if you want.