Upload firmware to many controller ICs at once
What you are asking about is called a gang programmer. Commercial ones tend to be “professionally” built with commensurate pricing. You need a circuit for each MCU since errors are possible on each.
There is this guy who used a bunch of USB programmers and some batch files to accomplish the task, but for only 100, I would just do them one at a time. You could also purchase the chips pre-programmed or have your assembly house do the testing and programming as one step.
If you are making many thousands, you could design a test/programming fixture and test rig, but probably 5-10 at a time would make more sense than 100. Considering the handling time you get diminishing returns.