A full-size upright and a concert grand have exactly the same number of keys: 88 of them, 52 white and 36 black, running from A0 at the bottom to C8 at the top. So if you came here to find out whether a grand gives you more notes, it does not. What it gives you is a different instrument wrapped around the same keyboard.
Here is what actually changes, and when it is worth caring about.
Why 88 keys, on both
Eighty-eight became the standard in the 1880s, when Steinway settled on that range and the rest of the industry followed. It stuck because it covers essentially everything written for the piano. Composers had already learned not to write past the edges of the instrument, and the extremes get so muddy and so thin that very little music goes there anyway.
You will meet smaller keyboards with 61 or 76 keys. Those are portable keyboards and stage instruments, not acoustic pianos, and the missing notes live at the two ends you use least when you are starting. If you want to know what the 88 are called and how they repeat, the beginner guide lays the pattern out.
The real difference is geometry
A grand lays its strings and soundboard out flat. An upright stands the same assembly on its end so it can sit against a wall. Almost everything people notice follows from that one decision.
Laid flat, the bass strings can run much longer, and a longer string produces a richer, better-defined low note. That is the reason a concert grand is about 2.7 metres long. It is not showmanship, it is the bass. The soundboard also gets far more area to move air, which is what people mean when they call a grand fuller or more open.
Stood upright, the strings have to be shorter, and the bass strings often get angled across each other to buy a little extra length in a smaller box. Uprights come in a range of heights, roughly 91cm for the shortest spinets up to about 152cm for a full-size professional model, and height is the single best predictor of how good one sounds. A tall upright can be genuinely excellent. A very short one is fighting physics.
The action, which matters more than the tone
This is the part people underrate. In a grand, the hammers lie horizontally and fall back under their own weight, and the action includes a repetition lever that lets a hammer re-set before the key has fully returned. The practical effect is that fast repeated notes come easily and very quiet playing is easier to control.
An upright's hammers travel sideways, so they need springs to come back. Repetition is a little less willing and the quietest end of the dynamic range is harder to place precisely. For most playing you will not notice. In a fast trill or a repeated-note passage you will.
Pedals differ too. Grands generally give you a true sostenuto in the middle. Uprights often use that middle pedal for a practice mute instead, which drops a felt strip between hammers and strings so you can play at 11pm without a conversation about it.