Breasts extremely dense. Microcalcifications in upper outer quadrant left breast the posterior one third are loosely grouped on the cc. How bad is this?

Hard to say. Calcification almost always causes patient confusion and concern. You are not alone. However, without further description about the appearance of the calcs, distribution, absence or presence of other calcs, etc, its impossible to give you a fair interpretation. In general, management is either 6 month follow up or stereotactic biopsy. I recommend talking to the radiologist who read the mammogram.