This might be a silly question 
But how can a 34" scale length have both 20, 22 and 24 frets without changing the length of the bass?
(I’m a beginner playing a Yamaha TRBX174)
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There is a couple of things to keep in mind here: while the overall length of the string is important, the tuning of that string can be changed (within reasonable limits) by tightening or loosening it. This is why your E-string can be tuned down to D or even C, but it starts to become very sloppy at some point.
The spacing of the frets is adjusted to the length of the string, such that the 12th fret exactly sits at half the string’s length, and the other frets are placed accordingly. The 12th fret is always at half the string’s length, regardless of the overall absolute length of that string.
Finally, the number of frets is often determined by the design of the neck in conjunction with the design of the body (i.e., if you have more frets, you need to allow access to the frets; or, if you want more frets, you might have to use a design that allows the fretboard to extend further down, and thus you need to place your pickups further towards the bridge; and so on…)
I have 34 inch basses with 24 frets, and 35 inch basses with 21 frets - I can certainly notice the difference in fret spacing and where, e.g., the 12th fret sits with respect to the body, or where my muscle memory thinks it ought to sit 
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Even though the overall length of the strings, measured from the nut to the bridge, is fixed, the length of the neck and the size of the body may vary. A 24-fret bass typically has a longer neck, and a shorter body to compensate. Compared to a 20-fret bass, the first 20 frets will be in exactly the same position on both basses. The 24-fret bass just has four additional frets at the “high” side of the strings.
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To make it simple.
Bigger (longer width) body + shorter neck = less frets
Smaller (shorter width) body + longer neck = more frets
Body + neck make same length of 34’’ between bridge and nut in both cases.
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Many times, 24-fret basses have deeper cutaways to allow access to the upper frets. The overall body length wouldn’t necessarily be affected between a 20-fret or a 24-fret. But the fretboard lengths would be.
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@Mike_NL’s and @Growl’s explanations are easy to visualize, but I would try to avoid seeing body and neck as two parts, where one is longer and the other shorter (as needed). In essence, it is the fretboard (or fingerboard) that decides the number of frets and it can be constructed/placed differently with respect to the body and neck.
Here is a 24 fret fingerboard, but the ramp has the same height and since it’s a fretless anyway, you can keep playing (if you can place your fingers).
Here is a bass with 27-ish frets:
(Note: they are both 35 inches in scale, but that is not my point here )
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If it take my TRBX174 and pull out 4 frets, it’ll have 20 frets but the length of the scale hasn’t changed and the remaining frets are still in the same place
Here’s a calculator you can play with to see how scale length and number of frets work Fret position calculator - StewMac
PS never start a question by suggesting that you think it might be silly/stupid.
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This is very informative thanks
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I think the simplest way to describe this is scale length does not equal fretboard length. Additionally, there are other variables here, as the others mentioned.
For a given scale length, all of the frets must be in the same position along the string, but there’s a lot more you can play with around fretboard length and end position relative to the bridge. The only things that must be static here are the distance from bridge to nut, and the position of the frets along the string.
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Huh.
You know, that makes complete sense, but I didn’t know that. Learn summat new every day.
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Physics, buddy, physics 
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Science!
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I keep saying that I’m going to build a bass and only put the top 5 frets on it; that’s all I need 
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Fretted 1-5, fretless the rest of the way!
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I didn’t know any of this either until i started learning bass
“The Inverse Proportion means that if we play 1/2 of the string, we get 2 times the frequency of vibration of the string. This means the musical note gets “twice as big”, making it become the same note, but one octave higher.”
I read this a while back and there’s a lot of math in music. Who knew? I mean everyone apart from me obvs.
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