Power Query M function - LOOP??

You can use List.Accumulate, like in the code below.

Remark: the step values are not really required to calculate the Count, so if you don't need it for something else, you can leave "step" out.

let
    Source = Table1,
    CountAndStep = List.Skip(List.Accumulate(List.Transform(Source[number], each if _ = null then 0 else _),
                                             {[Count = 0,step = 0]},
                                             (Result, Number) => 
                                                Result & {if List.Last(Result)[Count] = 0 
                                                          then [Count = Number,                           step = 0] 
                                                          else [Count = List.Last(Result)[Count] - Number,step = -1 * Number]})),
    AddedCountAndStepToSource = Table.FromColumns(Table.ToColumns(Source)&{CountAndStep}),
    Expanded = Table.ExpandRecordColumn(AddedCountAndStepToSource, "Column3", {"Count", "step"}),
    NewTableType = Value.ReplaceType(Expanded,Value.Type(Table.AddColumn(Table.AddColumn(Source,"Count",each 0, Int64.Type),"step",each 0, Int64.Type)))
in
    NewTableType

The example file is empty...

Make sure that you first have your table in Power Query, next it s used in my query (in this case "Table1", adapt to tour table name).

The number must be in column "number" (all lower case).