We have 9 windows. Assuming we will need 4 replacement springs per window as these are double hung with mobile top and bottom windows. The lower windows are easy to remove. Top, a little more challenging. Looking through your site looks like this style of block and tackle is no longer made/utilzed but there are replacement options.
Thinking it makes sense to start with a set of 4 to make sure this is the right size/fit for the set up we have. Also, to make sure I am talented enough to do this. I can reach out by phone next week if this is not clear.
Andrew