How to count each time some rows sum x number?

You can create a "conditional" running total using List.Generate; then use List.Accumulate to develop the Pallet List.

Data

let
    Source = Excel.CurrentWorkbook(){[Name="Table7"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"DN", type text}, {"QTY", Int64.Type}}),

//generate list of pallets
//replace >=80 with nulls to satisfy that condition
    qty= List.Buffer(List.Transform(#"Changed Type"[QTY], each if _ >=80 then null else _)),

//Generate a running total of the qtys that start over when there is a null 
// (ie an entry that was >=80) or when the running total reaches a Max of 80
    qtyList = List.Generate(
        ()=>[qt= qty{0}, idx=0],
        each [idx] < List.Count(qty),
        each [qt= if qty{[idx]+1} = null then null 
                    else if [qt] = null then  qty{[idx]+1} 
                    else if [qt] + qty{[idx]+1} <=80 then [qt] + qty{[idx]+1}
                    else qty{[idx]+1},
                idx = [idx]+1],
        each [qt]),

//generate the Pallet list by incrementing the list whenever the qty entry = the entry in the qtyList
//Note that the seed has to be {0} as {} or {null} + any number => null
//So we make the seed a {0} and remove that first entry when done generating the list.
    palletList = List.RemoveFirstN(
    List.Accumulate({0..List.Count(qty)-1},{0},(state,current)=>
        if qtyList{current} = null then state & {null} 
            else if qtyList{current} = qty{current} then state & {List.Max(state)+1} 
            else state & {List.Max(state)}
    ),1),

//add to table
    tbl = Table.FromColumns( 
    Table.ToColumns(#"Changed Type") & {palletList},
    Table.ColumnNames(#"Changed Type") & {"Pallet List"})

in  
    tbl

Results