I made a plant for ginger beer the other day and ours says to put in '8 sultanas'. Why is it so specific? Can't I put in 7 or 9 sultanas instead?

Recipes have to give some guidance or they won't work at all.  If you tried to make bread with double the recommended water, it will never bake into something worth eating.  Once you have some experience, you can start tweaking the recipe.  Most cooks do this.  I always double the spices in my pumpkin pie, because that's how I roll.