Looking at a microwave oven transformer it makes 220V into 2000V, so roughly 1:9 ratio.
Why then doesn't it have just one turn of wire on the input side and 9 turns of wire on the output side?
Instead, it has hundreds of turns on the input and what seems like thousands of turns on the output. Why is that?
Each winding on a transformer is an inductor. If the other winding(s) on the transformer are open-circuit (no current can flow), then this inductance is all there is to oppose current flow. If this inductance is small, a large current will flow, and if the inductance is large, a small current will flow.
This "magnetizing" current flows regardless of whether there is any load on the other winding(s) of the transformer. Therefore, for efficiency, we want to minimize its magnitude, which means we want a relatively high inductance. That's the fundamental reason we need a relatively large number of turns on the primary of your MOT. For a medium-size transformer, this will be on the order of 2-3 turns per volt.
The magnetizing current, being the result of inductance, has a phase angle that lags the applied voltage by 90° — purely reactive. However, any power drawn from the other winding(s) appears on the primary side as an in-phase current (0° phase angle) that simply adds to the reactive current. In general, you want the magnetizing current to be no more than 5% to 10% of the full-load in-phase current.
The transformer core can only support a maximum flux density before becoming saturated.
As the flux is swung between negative max and positive max, it will induce a certain maximum number of volts per turn in any winding, no more. Any excess of applied voltage over this maximum induced voltage will cause a large and damaging current to flow. A lower applied voltage will cause the core to operate at a lower flux.
Typically, MOT-sized transformers have sufficient core area to give about 1 volt per turn when operated right at saturation, so need in the order of 120 or 230 primary turns depending on where in the world they have been designed for.
MOTs are something of an outlier in transformer design, being operated right at, or even slightly into, saturation. This is because they are fan-cooled, built down to a price, and the operating costs do not figure in the design. The core area is therefore skimped to draw a 'not quite dangerous but still very large' magnetising current, in the same ballpark as their load current. All other transformers tend to be operated sufficiently below saturation that they draw less than 10% of their normal load current as magnetising current, so run very cool when off load.
