... to double... . Short answer: That's how it is said. Sorry. Surely you didn't think that English would be logical and easy :( . The number of tourists visiting is expected to double. - as you suggest. Tourist numbers are expected to double. . If you'd like to rationalise it you could think of a place such as Disneyland, with multiple gates, or multiple days and months. So we have figures for tourists entering each gate, each day, each month. Possibly we count locals and visitors, guys and gals, adults and children, separately as well as combine them. The numbers are expected to double. .

It means the same........visitor numbers....and the number of visitors. It is referring to the amount of visitors.