How much blow in insulation needed for 12' x 14' attic space?

It is difficult to give an answer in terms of number of inches or total volume / number of bags required.

For one thing, it depends on how deep the insulation is, i.e. it takes twice as much to make it 6 inches deep as it does to make it 3 inches deep. The deeper / thicker the insulation, the more it insulates. This is usually expressed as an "R-Value" where a higher value is more insulation.

Note also that there are different kinds of blow-in insulation, e.g. fiberglass and cellulose. These have different properties and require different amounts to achieve the same thermal effect.

One easy way to figure this out would be to go to the store where you plan to purchase the insulation and look at the bag. The bag will have a chart that shows the square footage or dimensions of the space to be insulated and the number of bags required to achieve various R-Values. If you know the company who manufactures the insulation, you can check their website. It is quite likely they have a similar chart of calculator as well.