First you need to know FXO vs FXS.
An FXO port (red module) lets Asterisk use and answer an existing telephone line that is provided by a telco. On a normal pbx this is called a Line or CO port, becasue you plug the phone line into it.
an FXS port (green module) provides dialtone and ringing to one or more telephones, this is an extension port.
So for each analog line you want * to deal with you need one FXO port, and for each ANALOG extension you want * to deal with you need one FXS port. You can also get VoIP phones, which are generally a better way to go.
The first link is a X100 clone. Digium used to sell a 1-port FXO card called the X100- it was just a rebadged voice modem that they’d written a driver for. X100’s (especially the ‘clones’) often work but then again they are often the cause of a wide variety of strange issues, things like heavy, unsolvable echo, ring/callerid detection problems, and other general strangeness.
The X100 was soon phased out and replaced with the TDM400 card. The TDM400 is a 4 port modular device- each of the physical ports can be enabled as FXO or FXS by adding the appropriate module to the card. As stated, FXO modules are red, FXS modules are green. The TDM400 is available in every possible configuration of modules. You can buy it with only one module to start and then add more modules later as you need them.
Now to set up a basic PBX, you will need a TDM400 card with one FXO module per line and one FXS module per extension. FXS extensions will connect to standard analog telephones or analog cordless phones. Note that FXS ports will typically NOT talk to analog PBX system phones- so if you have phones from your old PBX they probably won’t work.
However I’d recommend IP phones. IP phones connect to * via SIP over ethernet and have useful buttons like transfer, conference, hold, etc. With analog phones you have to do flashing and star codes.