True. A lens at f1.4 will allow double the amount of light compared to f2. ( varying by a factor of root two )
The aperture you want depends on what you are trying to achieve. Apart from varying the amount of light, a smaller aperture size gives for more depth of field and more things will be in focus as well.
A larger aperture size gives more light, and the option of using a faster shutter speed to freeze motion; likewise a smaller aperture can reduce light, allowing you to use a longer shutter speed to create motion effects like a silky flowing river without the risk of overexposure. These are just some basic examples. You will need to read up on the interplay of shutter speed / aperture / ISO and metering concepts to have a meaningful understanding of exposure. There are many resources online.
Lenses with large aperture size / smaller f numbers are generally more pricey / heavier than their counterparts. Get what you need and within your budget.