The default configuration of an Amiga 2000 has a single internal floppy drive. Can I add a second internal floppy drive? Can I add a third or fourth drive too?
Yes, you can add two internal drives. However, although the Amiga supports up to four floppy drives, you cannot add a third or fourth drive internally - additional drives need to be connected to the external drive port.
To use two internal drives, you need to have the correct cable. The cable will have three connectors on it, two for drives and one for the mainboard. The mainboard connector will be at one end of the cable and will have pin 3 blocked off as a key to make sure it is not inserted the wrong way round. At the other end of the cable are the drive connectors, and there will be a twist in the cable between the two drive connectors. The twist will cover pins 4, 5 and 6 (effectively swapping pins 4 and 6 on the last connector on the cable).
Note that the Amiga internal floppy drive cable is different to the IBM PC floppy drive cable: the twist covers a different set of pins, making the cable incompatible with the Amiga.
You also need to examine the jumpers on the drives. They will have jumpers to select "DS0" and "DS1" (and possibly others, but they are not relevant to the A2000). Set your "DF0:" drive to have the jumpers for DS0 closed. Set your "DF1:" drive to have the jumpers for DS1 closed.
Then, plug the DF0: drive into the connector at the end of the cable, after the twist. Plug the DF1: drive into the connector at the middle of the cable, before the twist.
Finally, locate jumper J301 on the mainboard (it's close to the floppy connector). This jumper needs to be closed (i.e. a shorting block plugged into it).
To configure an A2000 with a single drive, leave the middle connector on the cable disconnected, and open jumper J301 (i.e. both pins exposed).
It is a tempting idea to try and fit a HxC floppy emulator (which can emulate two drives) as well as a floppy drive, giving a total of three internal drives. Unfortunately the mainboard floppy connector only exposes the "select" signals for DF0: and DF1:, so it is impossible to use more than two drives from that connector.