Answer:
Three.
The Mayans created and used at least three calendars.
1) The Haab. This was 365 day long with 18 months of 20 days each. 5 days were added at the end, these days were considered to be unlucky and could not wait for them to pass. They were aware that the year was closer to 365.25 days but this was not taken into account in this calendar and consequently it drifted out of synchronisation. (By about six hours a year)
2) The Tzolkin. This was a 260 day cycle also called the 'Sacred Round' and listed sacred days, omens and associations for guidance in planting, warring, sacrificing etc.
This was joined to the 365 day calendar to give a complex 52 year cycle called the Calendar Round. This period is the time it takes for the two calendars to match up and start on the same day again.
3) The Long Count. This is where the ignorance comes in and is used by people to say the world will end. It is not so unless you are a Mayan or believe as they did.
It is a bit more complex and is used to calculate long periods of time. (Hence the name).
The periods within it are;
20 kins = 1 uinal = 20 days
18 uinals = 1 tun = 360 days
20 tuns = 1 katun = 7200 days
20 katun = 1 baktun = 144,000 days
They then multiplied the baktun by 13 to get what they called the Great Cycle which is equal to 5,130 years. The best evidence suggests that the present Great Cycle started in 3114 BC and will end on 23rd December 2012.