Sure, if you completely ignore all of the extra circuitry for the on-chip network and cache coherency and everything else you would need. Transistor density is far from the limiting factor in the number of cores we can wedge into a single system.
If you assume that there is no cache (only on-die memory) and that memory is not shared between cores, things become much simpler and scale more linearly. Core-to-core communications and plenty of other details remain to spend man-years ironing out, but it seems like it would be possible to approach 64k cores or at least 16k.
MOS 6502, Number of Transistors: 3510
So in theory, a chip with 65536 MOS 6502 cores each with 64K of internal RAM (4Mb cache) could be made.