Can I get two tourist visas in a single calendar year in Thailand, and is the calculation based on the calendar year?

Andrew Campbell . . . when an Immigration official checks your touristic entries, he will check your entry history within the recent 365-days period. He does NOT calculate by calendar year

Obtaining a tourist visa is really an irrelevant question.

Immigration is cracking down on people staying long-term in Thailand without long-term visas. A tourist entry (visa exempt, tourist visa) are not long-term entries, so you cannot stay in Thailand long-term. Calendar year has nothing to do with it. Many people are getting questioned and warned for simply coming in for 60 days, getting the 30 day extension, and then leaving and returning. There's almost no chance you would be able to do this 4 times consecutively.

The general rule that many immigration officers seem to use to start causing problems is 90 days within the last 180 days, and 6 of the last 12 months.

There is no such limit but each Embassy makes their own rules on whether or not to issue you with a visa.