Bt restrict the box to receiving 2 multicast Ip channels simultaneously so effectively you could record 2 IP channels , or record 1 and watch another IP channel. So in IP mode two is the limit.
In aerial mode the box is able to receive 4 Freeview channels so up to 4 simultaneous recordings of the Freeview channels can occur. The same restrictions on IP multicast channels apply so 2 IP multicast channel can be recorded. So in aerial mode you could record up to 4 aerial channels plus upto 2 IP multicast channels so up to 6 recordings.