Also, what i do not like is your comment "1 951Ui-2HnD to use as “server” ". Its bad practice to relay on only 1 device in such implemetation. Consider VRRP or whatever suits your needs on main location.
Is the remote sites connected to different Internet ISP’s?
And what type of bandwidth are you hoping to achieve with 24 concurrent VPN connections?
A easy way,
Each remote site will have a different subnet. eg the HQ 192.168.0.0/24 and then site 1 192.168.1.0/24
You can set up a sstp server on the HQ 951 and connect that way.