How much potato to add to reduce saltiness

Let's talk potatoes

Potatoes are kind of bland & starchy on their own. If you boil potatoes & eat them with no salt, they just taste like nothing. Most "plain" potato preparations will use salt & a bit of fat to make the potato taste more like potato. On the other end of the spectrum, you can put a lot of salt onto potatoes without them tasting "too salty." Fast food french fries can have quite a bit of salt on the outside, and the starchy, bland inside will offset all that salt.

Potatoes to fix salty food

That last bit of the above paragraph basically explains why adding potatoes to a salty dish can work. Potatoes "take" a lot of salt themselves, so if you've put too much salt in a soup or curry, adding unsalted potato will equalize that as the potatoes "take on" and "absorb" some of that saltiness from the liquid they are in. Other bland, starchy foods work well as a "salt sponge" too--rice or pasta or even bread or flour tortillas. Potatoes have an advantage that they can either be broken up & kind of turned into a thickener for the sauce/soup/whatever, or they can be left in big enough chunks that you can fish them out easily and not include them in the final product--where rice, pasta, bread, and tortillas are more difficult to make disappear.

How much potato you need will vary, and in my experience there is no "1 potato per x quarts of liquid" formula, because there are just too many factors (including personal preference). The easiest way for me to estimate how much potato to add is to think in terms of "how much extra salt did I add?"

You mention that you probably added 1 teaspoon too much salt, so per my advice, you'd think backwards to "how much potato do I need to cook to directly add 1 teaspoon of salt to season it?" and go from there. You might be able to add less or need more than that guess--but that's essentially what you're doing.

If you were cooking 1 potato by itself, how much salt would you use on it? If you would use 1 teaspoon of salt on 1 potato, then you need 1 potato to try to "fix" your "1 teaspoon too salty" curry. If you would use 1/2 teaspoon of salt on 1 potato, then you need 2 potatoes to fix your curry. Practically speaking, you probably need several potatoes (or many potatoes if they are small) to offset 1 teaspoon of salt by themselves.

But it doesn't always work

There's always an exception. If everything in the dish has absorbed too much salt, potatoes might not be enough to fix it. Potatoes do really well at taking on the salt from the sauce or liquid that they are cooking in. However, if there are meat & veggies & other things that have already absorbed "too much" salt, potatoes might not do the trick.