What amplifier does the CX122 need?

It is a passive 4-ohm sub rated at 700 watts RMS, so pair it with a mono amp that makes close to that power at 4 ohms. A weak amp will clip and can damage the sub.

Does it come with an enclosure?

No. It is a bare driver, so you need to build or buy a sealed, ported or tube box. Boss lists all three as suitable.

Is one CX122 enough bass?

For most listeners, one 12-inch sub in a ported box gives strong low end. If you want more output or a dual-sub look, two can be wired to a suitable amp.

Can I use it with a factory stereo?

Only with an added amplifier and line-output converter. The factory head unit alone cannot power a passive subwoofer.

Is it good for sound quality or just volume?

It leans toward volume. Reviewers say it hits hard but lacks the accuracy of pricier subs, so it suits bass-heavy music and budget builds more than critical listening.