How would you punctuate this sentence how many stoplights are on Main Street if know?

Here are a few ways to punctuate that sentence, depending on the intended meaning:

Option 1 (Direct question):

* How many stoplights are on Main Street if you know?

This is a direct question, asking the listener to provide information if they have it.

Option 2 (Incomplete question):

* How many stoplights are on Main Street? If you know...

This is an incomplete question, implying a continuation that might involve a request for information.

Option 3 (Statement with a question):

* How many stoplights are on Main Street, if you know?

This is a statement about the number of stoplights, with a question about whether the listener has that information.

The best option depends on the intended meaning and context of the conversation.