Creating a customer number based on the values of another column

Try this:

let

//change next two lines to reflect actual data source
    Source = Excel.CurrentWorkbook(){[Name="IDtbl"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"ID", Int64.Type}}),

//You don't indicate where you get the first value from
//   so you'll have to code it somehow.
    #"First UCN" = 38294,

//Generate list of UCN's and associated IDs
    UCNs = List.Generate(
        ()=>[ucn=#"First UCN", id=#"Changed Type"[ID]{0}, idx=0],
        each [idx] < Table.RowCount(#"Changed Type"),
        each [ucn=if #"Changed Type"[ID]{[idx]+1} = [id] then [ucn] else [ucn]+1,
                id=#"Changed Type"[ID]{[idx]+1}, idx = [idx]+1],
        each {[ucn],[id]}),

//convert to a table
    tbl = Table.FromColumns(
            {List.Alternate(List.Combine(UCNs),1,1,1)} &
            {List.Alternate(List.Combine(UCNs),1,1,0)},
            type table[UCN=Int64.Type, ID=Int64.Type])
in
    tbl