How many solutions are there in the equation y= -2x + 4?

You need to learn the words. The given equation has two unknown values, x and y. Either or both can have any real value. (Imaginary equations are a separate subject.)

"Solution" is when you have one value and you calculate the other. There can be any number of given values, so there are any number of solutions. It is common to call that "infinite solutions". That is not really what "infinite" means, but as long as it causes no confusion it's ok.

All the unknowns, x and y, are first power so this is called a linear equation. That means when you plot it you get a straight line, like this:
www.wolframalpha.com/input/?i=plot+y%3D+-2x+%2B+4