Maybe easier to estimate is the number of mathematical research papers published. Most of those will contain at least one original theorem, many of those will contain multiple theorems.
Found on-line in the Canadian Encyclopedia: Canada produced 73 mathematical research papers per year per million of the population in 1990. US and Holland followed with 47 mathematical research papers per year per million of the population.
From the Association of Research Libraries ( http://www.arl.org/sc/subversive/viii-a-researchers-perspective.shtml ):
The exponential growth in mathematical publishing has interesting implications. Adding up the numbers in [MR] or simply extrapolating from the current figure of about 50,000 papers per year and a doubling every 10 years, we come to the conclusion that about 1,000,000 mathematical papers have ever been published.