yes, as far as I know the Digium 8 port card has the channels split into two blocks of four, it appears to use the same four channel modules as the TDM2400.
I use a Yeastar YSTDM800, these take four, two channel modules so they are more flexible re. the number of FXO & FXS channels. They also do a module with one of each port that connects through if the power fails, so that extension connects directly to the trunk if the power fails.
There are other makes, but I’ve only use Digium and Yeastar cards.
You could also consider digital VoIP phones for extensions, or adding phone ports using ATA boxes, which just connect via ethernet and give a normal phone socket.
One analog phone line can only handle one call.
You can get a digital line from your phone company that will allow two or more calls, but often these cost more than extra analog lines until you get to a realtively large ‘line’ count.
You can of course add VoIP / SIP ‘lines’, there are many providers that will give you a normal Dial-in number that connects to your Asterisk system via SIP. Quite often these are free for incoming calls and cheap (or occasionally free) for outgoing calls.