The human capacity for storing long-term memories is

The term long-term memory denotes a relatively permanent and limitless storage part of the memory system. Thus memory includes knowledge skills and experiences
The human capacity for storing long-term memories is essentially unlimited.
Short-term memory on the other hand is less permanent and more limited in storage capacity.

I believe the human capacity for storing long-term memories is essentially unlimited.  Human brain consist of about a billion neurons. Neurons combine such that each one helps with many memories at a time, exponentially increasing brain's memory storage capacity to something closer to around 2.5 million gigabytes. Information entering the brain may enter the short term memory and some of it is transferred to the long-term memory.