How many Hard Drive slots does the G7 have?

You can usually find out about system capabilities by going to the Product sales site or going to its download page and looking at the Setup manual or the Service Manual.

Since I don't know which version you are looking for you might check the download site for this one.

https://www.dell.com/support/home/us/en/19/product-support/product/g-series-17-7790-laptop/diagnose

Storage

Your computer supports one of the following configurations

:•One 2.5-inch hard-disk drive

•One M.2 2280 solid-state drive

•One 2.5-inch hard-disk drive and one M.2 2280 solid-state drive

•One 2.5-inch hard-disk drive and one M.2 Intel Optane memory

NOTE: The primary drive of your computer varies with the storage configuration.

@Abdul-Hakim Badmus  asking about "the G7" isn't specific enough.  G7 is an entire product line that includes multiple models, sort of like "BMW 3 Series".  As of this writing, Dell has 3 separate G7 models on sale, namely the 7590, 7790, and 7588.  As time goes on and historical models accumulate, this will get worse.  If you want to learn about specs like this, you need to ask about the specific model you're interested in.  Or you can just look up the specs either on the product page or the documentation on support.dell.com.  For the latter, enter "G7" and then wait for the search results below to populate, then select the model you want.  Then click the Manuals & Documents section.  Typically there will either be a User Manual and/or a Setup and Specifications document, and one of those will have the specs of the system.  You'll also typically find a Service Manual there, which can be handy when you're ready to start swapping parts.