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Curry powder is strong, so start small and add gradually. For a soup that serves 4 people, begin with a scant teaspoon (roughly half a tablespoon) of curry powder, toast it well in a bit of oil or butter before adding the other ingredients. This way the flavor spreads better and you'll know right away if it's enough. If you're using curry paste, use even less - maybe half a tablespoon for the same amount, since it's already more concentrated.
Taste the soup when it's almost done and add more curry a little at a time if you think it needs it. Remember that the flavor intensifies a bit as it sits, so it's better to be conservative at first. If you accidentally overdo it, you can always thin it out with a bit of broth, or add a potato or two to dilute it. It also depends on how much curry you normally use in cooking - if you're used to spicy dishes maybe go a bit more generous, otherwise play it safe.
One thing that works really well with squash is adding the curry along with a little fresh or powdered turmeric (really just a pinch) and some ginger - the flavors balance better and the result is less flat.
Start with half a teaspoon for a medium pot and taste as you cook, adding a little at a time until you find the balance you like. Curry is tricky because the flavor intensifies during cooking, so it's better to start cautious than end up with an inedible soup. If you use curry powder it's easier to control the dose compared to paste, which tends to be more concentrated and aggressive. Once you know your palate, the recipe becomes an easy thing to dial in.
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