If we accept that there are only two corporate callings by God, then there will be found in Scripture only three dispensations of time that can be associated with these callings. This number is based on the distinctly separate times when God acknowledges one of the two callings. In the bible we can easily see these three: a Jewish dispensation, a Christian dispensation, and a millennium. An accessory principle to this is that God can/will recognize only one corporate calling at a time, that is, only one calling per dispensation we may be considering. Because it was the purpose of God to prove mankind’s fallen state using the Jewish dispensation, it was a miserable failure. The calling of Israel was not fulfilled during their dispensation, but was set aside by God at that time only to be taken back up and fulfilled by Him in the future millennium. That is why two callings produced three dispensations.

These chapters or articles detail the three dispensations of Scripture:

https://www.reintgenchristianbooks.com/chapter/chapter-three-dispensations/

https://www.reintgenchristianbooks.com/chapter/chapter-jewish-dispensation/

https://www.reintgenchristianbooks.com/chapter/chapter-ten-christian-dispensation/

https://www.reintgenchristianbooks.com/chapter/seventeen-millennium-dispensation/

https://www.reintgenchristianbooks.com/dispensations-differing-principles/

This following article elaborates on what dispensationalism is as a theological system and how it has been watered down over time:

https://www.reintgenchristianbooks.com/returning-classical-dispensationalism/