A person should change their maternity pad when it is full. The heaviness of the flow will determine how often a person needs to change pads.
In the first few days after giving birth, a person may need to change pads every one to two hours. As the flow tapers off, the need for changes will diminish.
The total number of pads a person needs will depend on:
how much they bleed at first
how quickly the bleeding tapers off
how long the discharge continues
General estimates for how many maternity pads a person will need to start off with range from 24-36. Since the nature of the discharge will change over time, a person may choose to switch from a more absorbent pad to a slimmer one.