The first CPUs contained around 2000 transistors. How many might you find on a CPU today?

The first CPU did not have transistors. They were not yet invented. The first computers used the electronic components available at that time. They included resisters, capacitors, diodes, and vacuum tubes (and wires). A vacuum tubes was a glass container with a wire assembly inside in a vacuum. It has a glowing filament (similar to that of an incandescent light bulb) that generated heat to make the tube function. Tubes would be replaced when the filament burns out after a while and their were many of these tubes in a computer generating a lot of heat.

Vacuum tubes were later replaced by transistors when the technology became available. Transistors used much lower voltage and less electricity. Much of the wires were replaced with printed circuit boards where the wires were printed, painted, or etched on a board. The wire connectors of transistors, diodes, capacitors and resistors were attached to the board circuitry.

Scientists later learned that they can eliminate the casing and wire leads of components and embed them directly into a circuit boards making everything smaller. They put components on both sides of a board, They used components instead of wiring through a hole in the board to connect the top to the bottom of a board. Circuit board layouts were redesigned eliminating wiring between connecting components. That with the technology to have thinner wiring and smaller components led to ability to put the parts of a cpu into its own sealed board small enough to be called a computer chip.

The first CPU chips were only CPUs with much supporting circuitry around it. Today much of the supporting circuitry has been incorporated into the CPU chip and the instruction set supported by the CPU has been greatly enhanced requiring even more components. A current CPU has millions of transistors and other components incorporated into a single chip and a CPU chip can have multiple CPUs in it.