Spanish smoked paprika is powerful stuff that can enhance or overpower a dish. How do I judge when, how much, and which type of paprika (Spanish, Hungarian, sweet, hot, smoked, etc.) to use?
If a recipe didn't specify which type of paprika to use (and I've never seen one that didn't) I would default to a 'sweet mild' paprika. Sweet refers to 'not chilli hot' rather than anything to do with sugar. Smoked paprika is a very particular ingredient used in very few cuisines so I wouldn't think of it as just a variant or substitute for the other paprikas.
The other clue about which to use will be in the rest of the recipe. Is it a spanish or basque recipe? Then smoked is likely what they mean. Is it Croatian/Serbian? Then it is sweet mild. Does the recipe include chilli as a spice separately? Then they likely won't be asking you to double up by using hot paprika too, use mild. The quantity the recipe calls for will also be a clue. Smoked & hot paprikas are very strong, so I'd expect to see a pinch or 1/4 teaspoon listed in most cases, whereas mild paprika is often used to add colour as well as flavour so you might see greater quantities used.
Your own taste determines it. Seriously, the amount and type you use depends on what you are accustomed to. What is overwhelming for one person is underseasoned for the next.
Ideally, the people who write down a recipe will know that and write down which type of paprika to use. But if they didn't, there is nothing you can do but guess, taking your personal taste into account. If there is one type you always use, start with this and change next time if it didn't work. If there isn't, you have to imagine the taste of the dish made with each type of paprika you have, and take the one which will work better. Such imagination is a skill you have to develop from years of cooking and tasting, and is not perfectly exact. But it's still the best you can do. There is no rule for taste.