How can I see how many customers we acquire each month?

You can check out the following three Metrics depending on what suits your needs best.

New Users = # of people sign up & create an account on day of sign up.

New Customers = # of people sign up, create an account & create an order on day of signup

New Orders = Number of first orders (only 1 per customers) on day of first order

You can set the time range, and group the data by month. You can see it visually on the graph, but if you scroll down it will be in a table, which you can download into a CSV.

You can also segment any of these by product types, locations, tags, etc.