Which ports on a computer facilitate connecting the computer to multimedia devices?

It depends what kind of device you use. Most devices come with the required connection cables. For MP3 players, phones and most other devices you will probably use a USB port, which is very popular and practically all computers have them nowadays. Most have about 4 at the front and maybe another 4 at the back (8 in total). All computers are not the same, but generally computers will have somewhere around that number.

If you are talking about memory cards, you can sometimes get a computer with a built in card reader. If not, you can get a card reader that plugs into a USB port instead.