Maybe it would be helpful to look at how some other translations read to see how other scholars interpreted these verses. That is, does the "for I have many in this city who are my people" explain why Paul should "go on speaking and do not be silent" or does it explain why "no one will attack you to harm you."
In the first case, we could adequately write: "go on speaking and do not be silent, for I have many in this city who are my people." This must be how John Mac interprets the passage for his comment advocates that "my people" are the elect (who haven't been regenerated) and must therefore hear the word and hence be regenerated. I'm curious whether this interpretation is rooted in his understanding of Greek or in his bias toward Reformed Theology, namely election.
In the second case, we could adequately write: "no one will attack you to harm you, for I have many in this city who are my people." The implications of this reading are not necessarily those that bighaasdog has concluded. Instead, this could simply be revealing to Paul that there are people in the city who are believers, yes, and have enough influence in the city to prevent Paul's harm. In other words, Paul didn't really know at this point how many had believed from his own ministry or that of Silas and Timothy. God is assuring him that many others exist that he is not even aware of and Paul will be protected.
Okay, so let's see some other translations of this text, Acts 18:9-10. Pay close attention to punctuation becuase it helps us understand how translators thought that certain ideas or actions interacted with each other in a passage.
NASB
And the Lord said to Paul in the night by a vision, "Do not be afraid any longer, but go on speaking and do not be silent;for I am with you, and no man will attack you in order to harm you, for I have many people in this city."
NKJV
Now the Lord spoke to Paul in the night by a vision, “Do not be afraid, but speak, and do not keep silent; for I am with you, and no one will attack you to hurt you; for I have many people in this city.”
YLT
And the Lord said through a vision in the night to Paul, `Be not afraid, but be speaking and thou mayest be not silent; because I am with thee, and no one shall set on thee to do thee evil; because I have much people in this city;'
NIV
One night the Lord spoke to Paul in a vision: "Do not be afraid; keep on speaking, do not be silent. For I am with you, and no one is going to attack and harm you, because I have many people in this city."
The Message
One night the Master spoke to Paul in a dream: "Keep it up, and don't let anyone intimidate or silence you. No matter what happens, I'm with you and no one is going to be able to hurt you. You have no idea how many people I have on my side in this city."
Well, I guess The Message translation basically agrees with the second reading which isn't necessarily a consoling thought.

Most of the translators it seems don't really want to commit to what the second "for" clause modifies or explains. The NIV and Message step out on a limb, though, and seem to advocate for the second reading. In my mind the second reading also seems to be the most plausible, i.e. "no one will attack you to harm you, for I have many in this city who are my people".
Aside: I appreciate most of John Mac's commentary.
Blessings!
Lewis