A soft microprocessor (also called softcore microprocessor or a soft processor) is a microprocessor core that can be wholly implemented using logic synthesis. It can be implemented via different semiconductor devices containing programmable logic (e.g., ASIC, FPGA, CPLD), including both high-end and commodity variations.
Most systems, if they use a soft processor at all, only use a single soft processor. However, a few designers tile as many soft cores onto an FPGA as will fit. In those multi-core systems, rarely used resources can be shared between all the cores in a cluster, leading to Jan's Razor.
Jan's Razor: In a chip multiprocessor design, strive to leave out all but the minimal kernel set of features from each processing element, so as to maximize processing elements per die.
While many people put exactly one soft microprocessor on a FPGA, a sufficiently large FPGA can hold two or more soft microprocessors, resulting in a multi-core processor. The number of soft processors on a single FPGA is only limited by the size of the FPGA. Some people have put dozens or hundreds of soft microprocessors on a single FPGA.
A soft microprocessor and its surrounding peripherals implemented in a FPGA is less vulnerable to obsolescence than a discrete processor.