The most common type of computer?

The most common architecture of computer is "von Neumann", named for the 20th century mathematician John von Neumann, who came up with the basic arrangement where both instructions (program) and data are stored in main memory, which is read and written by the CPU.

The most numerous kind of computer chip these days is probably "system on chip" designs using an ARM CPU and various other parts such as the networking and the graphics processing all part of one chip of Silicon. This is the kind of chip found in almost all modern cellphones, even simple ones, as well as most tablets. Most PCs use an Intel or AMD chip using the X86 or X64 CPU design. There are roughly 5 billion cellphones and tablets in use, and about one billion PCs.

Both ARM and X86/X64 designs are classified as von Neumann architectures.