I am buying a bicycle soon. Are more gears better? How many gears should my bicycle have?

You first have to decide what sort of bicycle will suit your needs, you do this by figuring out how you plan to use it and then look at bikes that are designed for that kind of riding. More gears are better in the sense that fewer gears usually indicates that the bike has lower quality components. A bike with more gears will cost more, not because of the extra gears but because it will be built with better quality parts. That isn't the whole story however. Cheaper bikes are often sold only in one frame size that may or may not fit you. Better bikes are built in multiple frame sizes in order to suit different size riders. Your best course is to visit a few bike shops where the sales staff can help you decide on the best bike for you

More gears are not necessarily better and they won't allow you to go faster.

More gears allow you to maintain your pedalling speed over varying terrain. For the best in efficiency (so you don't get tired fast) you should try and maintain the same number of pedals per minute (cadence).

MUCH more important is to purchase a bike suited to your needs and sized correctly.

Please consult these five, simple, easy-to-understand words...
Go
To
A
Bicycle
Shop

Avoid Kmart, Target, Wally World or online sites like Amazon.com. REAL bicycle shops have professional salespeople that live, breathe, eat & sleep nothing but cycling. They can answer all your questions. Amazing!

More gears doesn't always equal more speed. But LOWER gears can mean climbing hills better. The better bikes today will use an 8, 9, 10 & yes...even an 11 speed "cassette" rear cog. Avoid anything with a 7 speed "freewheel". The best all-around bike I own for BOTH speed & climbing hills uses a 9 speed cassette with a range of 11 to 32 teeth and a 34 - 50 compact crankset.
http://www.masibikes.com/bikes/performance/alare-2013

WHERE & HOW will you be riding? Get a bike based on that. Read this article & watch the embedded You Tube video. http://www.rei.com/learn/expert-advice/bicycle.html

Forget about gears for now, and figure out what kind of riding you want to do. This will determine what kind of bike you will get.

You're either going to get some form of road bike, a mountain bike, a touring bike, a hybrid, or a BMX bike.

When you know what style of riding you want to do, then go to a real bike shop ~ not a department store ~ and get some help.

The number of gears a bike has isn't nearly as important as finding the right style of bike for you, the right size for you, and one that is of decent quality.

More gears doesn't equal more top end speed. What it offers is more ratios between the lowest to the highest gear. What I generally set up for people here in the mountains is a 3x8 what some would call a 24 speed. I put the following chainrings on 22,34,44 with a 8 speed 34-11 freewheel or cassette. Then I tell them to shift like this 1(1-4); 2(3-6); 3(5-8) for 12 non redundant ratios. Uphill 1(1-4), level ground comfort riding 2(3-6), level ground sprinting or down hill 3(5-8). Never use both shifters at the same time; don't cross chain 1(8) or 3(1). The shift points for the front chainrings are between (3,4) and (5,6) in the rear.

You should also get a bicycle computer with a cadence and heart rate meters on them. Then be in a gear where the Target heart rate stays the same. Cadence should also stay between 70-90.

You should also use a starting gear. On a 3x8 be in 1(1) before stopping for stops going up hill. Then use that as a starting gear. All other stops use 2(3).

I think you should atleast have 4 to 5 gears for better experience and in case you need to have a great security app, the best app would be LEO Privacy Guard v 3.0. as it can secure your device totally.

A 21 speed works just fine with me. Others may like more. I'm not in a race when I ride my bike, but others are. If you go to a bike shop and ask, also check a few more shops, some of those "salesmen" are just out to make a buck. Trying to sell you something you don't want or need. It's happened to me plenty of times. Just make sure you get a nice bike that you want, is comfortable, affordable and works for you.

they now make 27 speed bikes and just buy the one that your happy and comfortable with

What is important is the right bike for where you want to ride and one that fits you. Gears allow you to match your fitness lever to the wind and terrain. If you want it for fitness gears are the way to go. 21 speed bikes use the old freewheel cassette while the 24 speed and above use the newer freehub that is stronger.

Stay away from department store bikes they are cheap, only one size, and wont last long.

Its really down to how, and how much/far you intend to ride.
will you ride more than a few miles?
Will you ride in a varied terrain?
If the answer to those are "yes" then some gears will be beneficial. How many is open to debate.
IMO bikes begin to be versatile and user friendly at five speeds or more. My commuter has a nominal 21, out of which maybe 14-15 are usefully different. Most of the year I'd be quite happy with less - say maybe 10 different. But for winter riding - snow and studded tires - I need some more low gears.
My bike tops out - becomes hard to pedal - at about 25 mph/40 kmh,and will remain rideable at a decent pedal pace to about walking pace.
It's OK for Urban/suburban commuting, but not enough for serious touring.