The number of turns depends on how big the transformer is. The larger the transformer, the more the magnetic flux, so the fewer turns needed on the primary.
If you don't have enough turns, the core will saturate, which means that no more flux can circulate in the core, so there is no back voltage, and a huge current flows and blows things.
If you want to mess around with a transformer, mess around with the secondary. All transformers in one manufacturer's range of the same power rating have identical primary windings, so you can customise the secondary without worrying about its affect on the primary.
If you don't touch the primary, you are less likely to kill yourself, and you will have enough turns so there will not be a problem with too much primary current when there is no load.
The larger the transformer, the fewer the turns you need for a particular voltage. A 300W toroidal will give about 1/2 V per turn.
Years ago, you used to be able to buy transformer kits, containing the core, primary windings and former for the secondary. All the user had to do was wind the secondary.