To guarantee compatibility with non-PAE processors, the 32-bit versions of Linux Mint Debian come with a 486 kernel by default. This kernel does not support SMP, and as a consequence is only able to detect one core and one CPU. If your CPU has multiple cores, or if you have more than one CPU, simply install the 686-PAE kernel and reboot your computer.
If you have the 32 bits version, install the 686-PAE kernel to be able to address more than 3GB of memory. Or install LMDE 64 bits instead.