These are multiple modems which might be used by an ISP or by an organization that has a number of phone lines for dialing in and out. There are two types of modem pools: analog and digital. An ISP will use digital so that they can support 56k (V.90, V.92) modems at near maximum speed.
A modem pool could be a number of modems on the same card (such as an analog multi-port modem card) or many modems in an external chassis (something like an external modem). The modems may be analog modems similar to modems used for home/office PCs (can't send above 33.6k even if they are "56k modems"). They also could be "digital modems" which can send at nearly 56k (if you have a good line). The "digital modems" require a digital connection to the telephone line and don't use any serial ports at all. All of these modem pools will require that you install special drivers for them.
A "multimodem card" is short for "multiport modem card". Some put a hyphen after "multi": multi-modem or multi-port. An analog modem pool is just many analog modems (the common home/office modem) provided either on an internal plug-in card or in an external chassis. Each modem comes with a built-in serial port. There is usually a system of sharing interrupts or of handling interrupts by their own electronics, thus removing much of this burden from the CPU. Note that these modems are not "digital modems" and will thus not be able to use 56k for people who dial-in.
Here is a list of some companies that make analog multiport modem cards which plug into slots in a PC. 8 modems/card is common. The cards listed claim to work with Linux and the websites should point you to a driver for them.
Multi-modem Cards (analog, not digital):
"digital modems" are much different than the analog modems that most people use in their PCs. They require a digital connection to the telephone line (such as T1) and don't use serial ports for the interface to the computer. Instead, they interface directly to a computer bus via a special card(s) (which may also contain the "digital modems"). They are able to send at near 56k, something no analog modem can do. They are often a component of "remote access servers" (RASs) or "digital modem pools"
The cables from the phone company that carry digital signals have been designed for high bandwidth so that the same cable carries multiple telephone calls. It's done by "time-division multiplexing". A single phone call in a cable is carried on two different channels, one for each direction. So the RAS must connect each such channel-pair to the appropriate "digital modem" that services that phone call. Such tasks are done by what is sometimes called a "... concentrator".
Now the digital signal received by a "digital modem" may really represent an analog signal which has been sent to it by an analog modem. One way to deal with it would be to convert it to an analog signal and then put that thru an analog modem to get the digital data sent by the analog modem. But why do all this work? Since the signal is already in digital form, why not process it digitally? That's how it's done. The digital signal is processed and converted to another digital stream of bytes which represents data bytes sent by the analog modem. A "digital signal processor" (DSP) is commonly used for this task. A CPU could also handle it but it would be heavily loaded.
Likewise, a "digital modem" must handle sending digital signals in the opposite direction from a RAS to a digital telephone line. Thus it only makes digital-to-digital conversions and doesn't deal in analog at all. It thus is not really a modem at all since it doesn't modulate any analog carrier. So the name "digital modem" is a misnomer but it does do the job formerly done by modems. Thus some "digital modems" call themselves "digital signal processors", or "remote access servers", etc. and may not even mention the word "modem".
Such a RAS system may be a stand-alone proprietary server, a chassis containing digital modems that connects to a PC via a special interface card, or just a card itself. Digi calls one such card a "remote access server concentrator adapter". One incomplete description of what is needed to become an ISP is: See What do I need to be an ISP?. Cyclades promotes their own products here so please do comparison shopping before buying anything.