The limit per withdrawal is determined by the bank of the atm and is often 10,000 pesos, the withdrawal limit per day depends on the bank your account is at, I set mine at usd1000, so I can withdrawal 43,350 per day if I want at today's rate of 43.35. The exchange rate used by the bank of the arm is the rate they buy dollars at and often extra charges apply, scotia bank for example charges 3% per withdrawal and no extra fee, others charge a % and a flat fee or only a flat fee.
At scotiabnk I noticed the rate they give you on Monday morning (and probably the weekend) at the atm is worse than the Friday before or the rest of the Monday, I guess to anticipate any abrupt rate change over the weekend. I'm not 100% sure if that gets fixed retroactively later during the day.