You asked how many rounds.. All you can carry if need be.
I only carry 10 in one clip. but you see, I no longer live near the southern border, If I did I would look like a swat member. I have a friend who has a ranch in Del Rio,, I haven't talked with him in ages, but he said he quit going there at all! lives in Austin now. As the sign I posted in Arizona said,, it is dangerous, the border is porous and there are alot of problems down there.
I am pretty sure you can't own a weapon in Mexico, or maybe nothing above a .22
But look at this mayor there, just on the other side.
Maria Santos Gorrostieta Dead: Former Mexico Mayor Found Tortured, Murdered In Michoacan