Create visit number for each customer

You can use a query to do this like (replace Table with your table):

let
Source = Table,
#"Grouped Rows" = Table.Group(Source, {"Customer_Id"}, {{"Rows", each _, type table [Date=date, CustomerId=number]}}),
Custom1 = Table.TransformColumns(#"Grouped Rows", {"Rows", each Table.AddIndexColumn(_, "Visit_Number", 1)}),
#"Expanded Rows" = Table.ExpandTableColumn(Custom1, "Rows", {"Date", "Visit_Number"}, {"Date", "Visit_Number"})
in
#"Expanded Rows"

P.S. this assumes the table is sorted by date. If not you will need to sort it that way first

The following measure will do this in DAX if you want to do it that way. Just depends on where you need it. On a very large data set, the Power Query method will cause a longer refresh, but depending on your model, a DAX measure may be slower to the end user. Anything under a few hundred thousand records though I doubt anyone would know the difference.

Unlike the PQ method though, this doesn't require anything to be sorted first.

Visit Count = 
VAR ThisCustomer = MAX('Table'[Customer ID])
VAR ThisDate = MAX('Table'[Date])
RETURN
CALCULATE(
    COUNTROWS('Table'),
    FILTER(
        ALL('Table'),
        'Table'[Customer ID] = ThisCustomer && 'Table'[Date] <= ThisDate
    )
)

EDIT: This is slightly cleaner.

Visit Count = 
VAR ThisDate = MAX('Table'[Date])
RETURN
CALCULATE(
    COUNTROWS('Table'),
    FILTER(
        ALLEXCEPT('Table','Table'[Customer ID]),
        'Table'[Date] <= ThisDate
    )
)