Some people would say that you only need one prime lens, because you can take care of the framing by moving closer or farther. That's not true, but that's a different story...
It depends on your photographic preferences and habits. You have to find out which are your preferred focal lengths, then you have to look how you can serve that range best. I explicitely did not write "cover that range", because that's what you do using zoom lenses. If you prefer to work from 50mm to 120mm, for example, then you would get 50mm, 85mm and 135mm.
Hopefully, everyone knows that aperture and time values are based on a logarithmical scale. But you can create a similar scale for focal length, because twice the focal length will give you half of the horizontal and vertical view.
|-2.0||12.50||12mm||more common are 14mm|
|-1.5||17.68||17mm||more common are 20mm|
|0.0||50.00||50mm||considered to be the "standard" focal length|
|+0.5||70.71||for unknown reasons, 85mm (+0.75) is a common value|
|+1.0||100.00||100mm||100mm is a less common value for regular (non-macro) primes|
|+3.5||565.69||500mm and 600mm are common, to make twice the money|
There is an alternative wide angle series of 14mm, 20mm, 28mm and 40mm,
where 28mm and 40mm are less common.
I have no idea, why there are no 70mm prime lenses. Maybe because that is the "magic" cutover point between the 24-70 and 70-200 zoom lenses, so everybody already has two 70mm lenses? At least there is a 100mm f/2.0 portrait lens, but for non-macro lenses, 100mm is not very commom. It looks like the camera lens industry decided not to use the 70mm and 100mm values, and chose the 85mm mark (+0.75) between 50mm and 135mm as standard value.
In order to get a better overview, I put created a graph showing the available lenses by focal length and aperture.Prime Lenses for Canon EF
Because Canon has failed to keep their product line up to date, I have added hints about the availability of Sigma Art and Cine lenses.
In most markets, the "cheaper" prime lenses come without lens pouch and lens hood. The recommended lens pouch size always includes the matching hood. If you want to store your lens without hood - for whatever reasons - a smaller diameter can be used.
There is not much to say about lens hoods, except that you have to get the exact model. The item codes start with EW, ES or ET, where W is for wide angle, S for standard, and T for telephoto. The number usually indicates the diameter of the mount, no matter which type of mount is used. The following letters or roman numerals are to distinguish the different shapes.
There are only very few different lenses that share the same hood. One example of "hood-twins" are the EF 85mm f/1.8 USM and the EF 100mm f/2 USM, where both require the ET-65 III.
Padded lens cases are excellent protection, but they add a lot of space. If you want to store or pack your lenses closer together, the original lens pouches are a very good option.
The item codes are easy to understand: LP means lens pouch, the stronger zippered versions for the telephoto zoom lenses start with LZ. The number indicates diameter and length in centimetres.
There are a few special sizes. The current extension tubes, for example, use faux leather pouches "CASE EF12 II" and "CASE EF25 II".