A lot will depend on how much you like to press the shutter button. I always have at least 2 rolls of each type of film I shoot (50, 100, 400iso B&W, and 100iso color, all 120) and then several extra of my most used films. So, I rarely embark anywhere without at least 8-10 rolls for the smallest day outings. When I am going on holidays, I plan on 3 rolls a day for a minimum although I have never shoot that many on a regular basis; this allows me to shoot 5 rolls one day, 1 the next and still be okay on average. Consider the expense of the trip (flights, hotels, food, time) and the film is one of the cheapest aspects of traveling; I personally would hate to miss shots because I was conserving film given what it cost me to get there in the first place.
The problem with buying film on your trip is: (1) will they have 120 film (it is getting harder to find without sometimes going to multiple stores which takes valuable time) and (2) will they have your particular preference for film (or just 800iso unrefrigerated generic film)? The problem with taking film is: (1) I still don't trust airport scanners and some airports refuse to allow the X-ray bags to be used and (2) many airlines now have weight limitations which prevent you from taking as much gear as you want.