12V/5A power through DB9
There are some DB9 connectors with 5A rated pins. Most are 3A rated pins (or higher) Assuming you design for the lower standard your options are use a higher voltage and use a switching regulator to bring it down to the Voltage/Amperage needed, or... use multiple pins. For 5A on a 3A contact, you'll need 4 pins, 2 V+ and 2 Gnd. Just ensure everything in the path can handle the current.