what percentage of people travel with their own mat when going to retreats around the world?
I do not think I can tell you %, but I would buy a cheap simple mat at the country of retreat and live it there, since varying it back and forward can be troublesome.
I brought my own mat on the first trip I made to study yoga, and it was nice to have extra padding beneath the mat I was given. The second time I went for a class, I was lazy and did not bring one, and wish that I had, as I did not have one to practice at my rented apartment with. When I leave again for a class next month, I will bring it, and will probably never even use it anyway. To each his own!