For most types of rice, you'll always use a ratio of 1 cup rice to 2 cups water. When the water is gone the rice is done. Some rice like arborio for risotto take even more.
A lot of people have strong views on the right ratio of water to rice (and I will admit this post is a bit hypocritical given how long it is), but ignore that while their ratio may work when cooking X cups of rice it often doesn't work when cooking 2X cups of rice. Meaning that such ratios don't fulfil their purpose as ratios, they're just an unscaleable recipe for a fixed amount of rice.
For instance, I'm sure a ratio of 2:1 works fine for you if you're cooking 1 or 2 cups of rice, but if you're cooking 4 cups of rice are you going to add 8 cups of water? I hope not, because I can guarantee you will get porridge. Why? Because there are two processes going on which use the water you've added (absorption by the rice and evaporation). The vast majority of rice absorbs about the same volume of water when cooked properly, but you need some extra water to account for the evaporation while the water is hot for the dozen minutes it takes to cook rice. But the rate of evaporation isn't dependent on how much water or rice you have, it's the surface area exposed to the air (which is the same for most rice cooking vessels). So using a simple ratio is incorrect -- one of the processes depleting the water during cooking does not scale with the amount of water.
In fact, your comment about risotto confirms this view through your own experience -- you have to add more water because a pan (which is what most risotto is cooked in) is wider than a pot and evaporates water more quickly (if you don't believe me, compare how long it takes to thicken sauces or evaporate a fixed amount of water between pots and pans). If you try cooking risotto rice the same way as normal rice you'll find it absorbs a similar (though possibly slightly more) amount of water.
This is why most people from Asian households will tell you that they don't use ratios to calculate how much water they need. They fill up the water to the level of the rice and then add enough water such that when they put their index finger vertically and touch the rice the waterline reaches the last knuckle on their finger (about 2cm). If you something more like a ratio, it's about a 1:1 ratio plus an extra cup of water -- but the amount will somewhat depend on the diameter of your cooking vessel.
Rice cookers are also covered vessels (though they have a small vent), so even if you are cooking rice on the stove evaporation would have a similar impact.
One way to check might be to try to cook the same mass of water as though it were rice and see how much water you've lost through evaporation. 250mL (1 cup) of water really isn't that much liquid to lose in ~20 minutes during cooking -- if you've cooked soups before you may recall that uncovered soups will lose far more than 1 cup of liquid in ~20 minutes.
