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

Depending on what suits your needs best, the three following Metrics will be an immediate help. Each of these you can search at the top of the Peel Menu

New Users is the # of people sign up and create an account

New Customers is the # of people sign up + create an account + create an order

New Orders is the # of first orders (only 1 per customers) on the day of first order. Note that a customer could be involved in multiple new orders but only count as one customer.

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. to pull out further.