There are several possible uses for plastic tubing in the home fermatorium.
Many hobyists can get by with just one length of tubing. It should be about five feet in length. It is primarily used for racking (transferring), and bottling.
If cost is an issue, you will get along fine with just one tube. If you are the 'gadget freak' type, you may want to get more tubing and cut it to length for several different uses. you could have one for racking, one for bottling, several for kegging and more as you add additional equipment to your system.
As dicusses in the page on stoppers and airlocks, you may also choose to purchase larger diameter tubing to use as a blow-off tube.