It's definitely a bug. I've tried to run a 4 total tourists cities. Every time using a slightly different strategy. I've payed attention to all factors - like trafic, crime activity, garbage, microbs, etc.
The real issue though is just next - after some period of time tourists just stop coming.
Actually its not entirely true - they are coming. In fact there are a lot of them - BUT the game doesn't recognise them as a tourists. That's weird, but they are displaying with white color on a tourism map, and they are always unhappy - saying smth like "there are no hotels" (when there are plenty), or "this city is boring" (when there are a ton of casinos and tourism buildings). This is like a desease - all tourists become "white", and they stop visiting places - the game doesn't recognise them as a tourists anymore.
Next time pay attention to it - you'll notice, a ton of people came from a train for example, the are all white on the tourism map, and they are all saying "city is boring" or "no hotels, leaving", and they just trying to leave as soon as they came - than you're screwed.
This is a crucial bug for me right now, because every other specialisation is pretty boring, and gambling/tourism is just bugged.
BTW the number of tourists isn't formed by the number of other cities in the region, or by their population - it's completely independent. Tourists are not inhabitants of other cities in the region, they are like a global resourse. So in theory you can have just 1 successful tourists city in the region.
Gambling city has become my obsession and a nightmare at the same time. I just can't get myself to play any else specialization anymore (played electronics, oil, trade - all piece a cake), and I can't play gambling while this bug is present.
Sadly, but for now it's enough Simcity for me.