Is the Samsung U7900 good for gaming?

It accepts 4K input but caps at 60Hz, so it suits casual console play rather than fast competitive gaming. Serious gamers should look for a 120Hz set with HDMI 2.1.

How many HDMI ports does it have?

Two HDMI inputs, plus one USB, composite and component inputs, Ethernet, optical audio out and Bluetooth. That is fewer HDMI ports than most rivals at this size.

Does it need a soundbar?

The built-in speakers output 20W, which is fine for news and casual viewing but thin for movies. Optical audio out and Bluetooth let you add a soundbar or headphones easily.

Yes. It uses a 200mm x 200mm VESA pattern and weighs about 30 lbs without the stand, so a standard mount rated for that weight works.

The TV and a remote control. A stand and wall mount are not listed, so check before buying if you plan to set it on furniture.

Is it worth it over a 120Hz TV?

If you mostly stream shows and want a low-cost 55-inch 4K set, it covers the basics. For sports, gaming or deep blacks, a 120Hz panel with local dimming is a meaningful step up.