"The number can be increased."
Translation:Antalet kan höjas.
Just to expand on Katsube's explanation: "att höja" is the infinitive, and adding an "s" forms the passive after an auxiliary verb like "kan" to give "kan höjas" for "can be raised/increased"; "höjer" is the present tense, and the passive is formed by removing the "er" and adding an "s", to give "höjs" for "is raised".
So one cannot say "Talet kan ökas genom att lägga till två.", by trying to say that one specific number can be increased by adding two? Thereby just adding the explanation how the/a mathematical number can in fact be increased.
And thanks for the fast reply!
Yes, nummer does not always, or even usually, mean the same as number. It does mean the same in expressions like phone number – which is also a good example of what kind of number nummer stands for. Ett nummer is an identifier or an ordinal number ('she finished as number three'), so it cannot be increased. Just like you could not increase a phone number (IP number, zip code, shoe number etc).
But when you say you have 'a number of reasons', or 'there is a large number of something', number cannot be translated as nummer, that is antal.
They’re not completely interchangeable, but can be. Öka is used for numbers and amounts, while höja is used for numbers and for literal raising of objects. For example, if you put a painting on a wall but you realise that you put it too low you can höja the painting, which means that you raise the painting, i.e. putting the painting higher up on the wall. You can think of it as öka meaning that a number or amount become bigger, while höja is to make a number become a higher number or an object is moved to be higher up.
Btw, minska (reduce) is the opposite of öka, and sänka (lower) is the opposite of höja.