The Relationship of the many lords and the one Lord in 1 Corinthians 8:5-6

1 Corinthians 8:5

New Revised Standard Version

5 Indeed, even though there may be so-called gods in heaven or on earth—as in fact there are many gods and many lords—

In 1 Corinthians 8:5, Paul spoke of the ''so-called gods''. In the very same verse, Paul sets up two locations of the ''so-called gods'': heaven and earth. 

 Paul says that there are so called gods....on earth. Who are these gods on earth? It's logical based on its context that they refer to the many lords because in the first century Roman colony Palestine, the Roman Emperors were gods on earth [1] 

We have biblical evidence/witness that the pagans worship the Emperors (the lords) as gods while on earth: 

Acts 12:21-23

…21 On the appointed day, Herod donned his royal robes, sat on his throne, and addressed the people. 22And they began to shout, “This is the voice of a god, not a man!” 

Yahweh himself was kyrios in Deuteronomy 6:4 (LXX). Yahweh wasn't the only kyrios (lord) in the ancient world as there was many lords (kyrioi). Yahweh was the "lord of lords" (Deuteronomy 10:17). In the Shema, Yahweh was the one kyrios (one lord) of the Isralites. Paul equates Jesus with Yahweh by applying to Jesus Yahweh's own role as the only kyrios in 1 Corinthians 8:6.

1 Corinthians 8:5-6

New Revised Standard Version

5 Indeed, even though there may be so-called gods in heaven or on earth—as in fact there are many gods and many lords— 6 yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist.

"lord'' in the phrases ''one lord'' and ''many lords'' have the same meaning of ''master'.  In the Shema, God alone is the lord/master: 

kyrios estin eis = [the] lord/master is one. 

In 1 Corinthians 8:6, eis kyrios is Jesus Christ. Paul's point is to equate Jesus Christ with the Lord spoken of in the Shema. Further evidence for this is that in the very same verse, Paul applies to Jesus the exact same prepositional phrase ''through him are all things'' which Paul used to God in Romans 11:36.

Notes

[1] The so-called gods on earth were the many lords whilst the so-called gods in heaven were the many gods. In Greco-Roman religion, the deities dwelling high up there were the high gods, such as the Olympian gods (e.g. Zeus, Athena etc.). In Latin, they were called deus. On the other hand, the so-called gods on earth refers to Emperors, human rulers or lords (Latin: dominus). These gods dwelling here on earth were called divus (another term for ''god'' usually ascribed to mortals). Also, Ceasar (circa  A.D. 27) was called ''Augustus'', a title applied to deities in the ancient world. 

The idols refers to both the gods in heaven and the gods on earth, and these gods on earth are referring to the many lords. Statues of Emperors were installed side by side the Greco-Roman gods in the ancient world. These Emperors were being worshiped as lord and god by the Greco-Romans. They offered sacrifices to them. Paul was arguing that these two numerically distinct figures: Jesus and the Father were closely linked in a similar fashion an Emperor (gods on earth called lords) was in union with the transcended god he bore in himself. The Emperor was called by the title or name of the god he was embodying and the people worship the  Emperor for he was a god and he was a god precisely because he manifested the transcended god visibly in himself. For instance, the Emperor Gaius (circa A.D. 37 -41) claimed to be the god Apollo on earth but his bad behaviour shows that he was not a true god. Philo said that the form of a god can't be faked like a coin as Gaius thought he could when he claimed to be the deity but his actions were just inconsistent with how a Paean (a true god) behaved ( Embassy to Gaius 110-14).