Hey....hope I can help
Adding strings to a bass guitar increases the range and allows you to to play more notes in any given hand position. Basically you can play lower than a four string (if you get a five string bass) and both higher and lower than a four string (if you get a six string bass).
A five string bass has a low B string underneath the standard four which is equivalent to the lowest B on the piano and can easily be down-tuned to reach notes even lower than whats on the piano.
A six string bass has a low B just like the five string but also has one higher string "a C string" which allows you to do all kinds of stuff with slap bass, harmonics, melodic playing, playing chords, arpeggios, tapping and in the most practical sense allows you to play lots of notes (both low and high) without having to physically move your hand up and down the neck.
There are also seven and eight string basses that have MANY different tunings but if you continue the pattern of typical basses....a seven string bass adds a low F sharp string below the B and an eight string bass adds a low C sharp string even lower. This C sharp is actually an octave below the lowest C sharp on the piano and is only used for effects because humans can't actually hear that low (we're talking 18 hertz)
A twelve string bass is like a twelve string guitar. ON a twelve string guitar each of the original six strings has a counterpart string right next to it tuned an octave up. A twelve string bass is a six string bass where each of the original strings is now simply TWO strings....one is at the original pitch and the other is an octave higher....this produces a unique sound with cool harmonies and natural chorus effects. You can also have an 8 string bass (very common actually) which is simply a doubled up four string bass or a ten string bass which is a doubled five string bass.
All you should know is that there are really MANY different ways of tuning basses and different numbers of strings you can put on them. I explained to you that a twelve string bass is a doubled up six string (and is pretty much played like a six string) but there are twelve string basses out there where each string is its own individual separate note that fits the same interval relationships as other basses. When you take this to the extreme...you can have basses with twenty something strings that cover the full range or a piano.
If you are looking for practicality, 90 percent if not more of bass players play four string....you should also know five string and its also nice to know that there are many cool things you can do with six string and eight string (doubled four) basses. Anything outside that is honestly useless outside of experimental music.
Many people chose to play four string because they dont need anymore strings and dont want to be worry about all the technicalities of playing 5-6 string instruments (muting for example)
Other bassists on the other hand base their whole playing and bass lines on the range of a five or six string instrument.
In summary....four string is the true standard.....five string is if you want to go lower and get a deeper bass sound ....and six string basses are really for people who want to push the bass out of its traditional limits and so some unique cool stuff.
double bass is the same things as standing bass. I have never even heard the term standing bass used. USe double bass or upright bass. Both the upright and the electric can go low and both can go lower than the piano but in the end the upright is an acoustic instrument and it won't be able to amplify below certain notes. With the electric bass you can easily drop way down below human hearing....you just need a big string....a powerful amplifier and a large speaker. So the electric can go down lower but if we are talking about useable pitches....its a tie. Seeing how the electric bass is an electronic instrument....there are really no limits with taking it below the piano or even below the range of a cathedral organ (even lower).
hope that explains everything.