How should I go about finding out how many 2 x 12 joist I need for a 61'-6" by 36'-0" floor with pier and post every 12 feet, 16" oc? Please let me know if you need more information.

If u have a girder located at 12 feet centers on the 36 foot side, divide length by 4 and multiply by 3 plus 1 times 3.

The total number beams needed would be 150.

Chuck