I assumed the stack size was similar to say a Roboport or Rocket Silo, but when I just looked at the provider chest containing the automatically produced reactors I saw 50 of them I'm not going to need anytime soon
You could just set a hard limit using a circuit connection on the inserter putting materials into the assembler. Then it will always stop after making the intended number.
In my mind, Steam is the eternal king of the railway.