how to add multiple documents at the same time in editable grid

Can you clarify what you mean by "at the same time"?  Your screenshot shows you have a document uploaded to each row already, which is the typical use case.

Could you explain a little bit your problem please? 

Hi [mention:cb656964172a4c65ba07f4d4ada5a0c6:e9ed411860ed4f2ba0265705b8793d05] ,

You can try the code below and let me know whether it works.

{
  a!gridLayout(
    label: "Editable Grid",
    labelPosition: "ABOVE",
    headerCells: {
      a!gridLayoutHeaderCell(label: "Document")
    },
    columnConfigs: {},
    rows: {
      a!forEach(
        items: ri!documentIds,
        expression: a!gridRowLayout(
          contents: {
            a!fileUploadField(
              placeholder: "upload files",
              target: cons!FOLDER,
              value: reject(
                fn!isnull,
                tointeger(split(ri!documentIds[fv!index], ";"))
              ),
              maxSelections: cons!MAX_SELECTIONS,
              saveInto: {
                a!save(
                  ri!documentIds[fv!index],
                  tointeger(save!value)
                )
              },
              buttonDisplay: "LABEL"
            )
          }
        )
      )
    },
    selectionSaveInto: {},
    validations: {},
    shadeAlternateRows: true,
    addRowLink: a!dynamicLink(
      label: "Add",
      saveInto: {
        a!save(
          ri!documentIds,
          append(ri!documentIds, null())
        )
      }
    )
  )
}

documentIds is a list of text string. In the process model, you can split the each index and iterate through each value individually