How much I can REALLY do with a certain amount of memory in my game?

Sorry if the title is not very defined. Let me explain a little better:

I am making a game for web. Now, I need to think about memory, but I have no idea where to start. So, just to create a example here...

Let's say the game (2d) has a system where it creates a lot of NPCs. Every NPC can buy items and have status of their own. To be more specific in the example: Items are a group of 5 strings, and the NPC has other 2 integer and 2 double/float variables, and one extra string for their name. 10 variables, just that here.

How many uniques NPCs it can be created simultaneously with an amount of 128mb or 256mb? (Not counting the rest of the game)

And if every item has a icon, a static image. Should I take the raw image size and calculate or when inside of Unity the image size changes?

How I do these type of calculations to have an idea of my limits?