One patent I have as inventor my institution issued first in Europe, and later in the United States; can I then say that I have two patents or just one?
If you have a patent in Europe and one in the United States, you do have two patents. That's how the patent system works.
Beyond that, the question just becomes what "can I then say" means. If you're speaking in the legal sense, it's more or less indisputable that you have two separate patents. On the other hand, if you're speaking in the marketing sense, that's a little more subjective. I would probably stick to saying "I have an invention patented in two countries," just since "I have two patents" is, while accurate, fairly misleading. Ultimately, it's just a matter of context. But I'd generally side on the more verbose, less misleading phrasing.