Serial number based on condition

There is a typo in row 3 of your sample data - I assume that should have said 30-12-23.  

Please provide sample data that fully covers your issue. Including a second customer.
Please show the expected outcome based on the sample data you provided.

Here is a stub that shows one possible implementation

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjLRNTTUNTJW0lEyhAClWB2gsKmuoRGmsLEBVmEDM10DoCEm6MJGukCEKWyCXdgSUzgWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Date = _t, #"Customer Id" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Date", type date}, {"Customer Id", Int64.Type}},"en-GB"),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Customer Id"}, {{"Rows", each _, type table [Date=nullable date, Customer Id=nullable number]}}),
    #"1111111" = #"Grouped Rows"{[#"Customer Id"=1111111]}[Rows],
    #"Sorted Rows" = Table.Sort(#"1111111",{{"Date", Order.Ascending}}),
    #"Added Index" = Table.AddIndexColumn(#"Sorted Rows", "Index", 0, 1, Int64.Type),
    AC = Table.AddColumn(#"Added Index", "NewIndex", each List.Accumulate({0..[Index]},0,(state,current)=>if current=0 or #"Added Index"[Date]{current}-#"Added Index"[Date]{current-1}>#duration(7,0,0,0) then 1 else state+1))
in
    AC

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done". Once you examined the code, replace the Source step with your own source.