A volume control on an instrument is a type of resistor, aka potentiometer (POT) or variable resistor (varistor) measured in Ohms.
At 100% 'open' it theoretically lets all the electrons flow and would be similar to having NO pot in the circuit at all, and at zero, or 100% closed, theoretically it should stop all the electrons from flowing.
So to answer your question, having NO volume control ON THE INSTRUMENT is the same as having the volume control at 100% open or '10' when going into the amp, and this has nothing to do with the volume setting on the amp.
Having a volume control on your instrument is for convenience, say for live performance when you want to mute the instrument or change volume level during performance, as opposed to having a VOLUME PEDAL, or having to reach over to the possibly inconveniently located controls ON the amplifier during performance.
More TECH details follow below: (not required for the answer to the question)
I say 'theoretically' above because there is always some 'bleed' when fully closed and as well as some 'insertion loss' (measured in decibels 'db') even when fully OPEN with any potentiometer. How much or how little depends upon the quality of the potentiometers in question.
You should also be aware of the difference between LINEAR (or logarithmic) potentiometers vs. AUDIO TAPER potentiometers. They each have a different 'curve', i.e., how much you turn the knob, and how much the sound level is effected proportionally. There are different viable uses for each type when being used for audio.
Also, there is no one single value for potentiometers, they need to be matched to the pickup(s) as well as the amplifier. There is a LOAD measure in kilo-Ohms from magnetic pickups, as well as the input impedance on an amplifier. There is no hard and fast rule, but 250k potentiometers will SOUND DIFFERENT than 500k or 100k potentiometers with the SAME pickups feeding into each. More or less treble, more or less warmth, depending upon the total source impedance load of the pickups themselves.
Also, a TONE control, or individual bass, middle and trble controls are nothing more than a potentiometer, wired in series with a specific capacitor. The capacitor acts as a filter to pass OR block certain audio frequencies.
Lots of this info can be had on the StewMac tech info, tech-tips, and tutorials pages, as well as other sources online.