Here is a script that adds two custom menu commands: "Freeze Formulas" and "Unfreeze Formulas". The logic is very simple: freezeFormulas replaces every formula in the spreadsheet with text value of the formula, preceded by triple backtick (randomly chosen string that doesn't normally appear in spreadsheets; you can use something else like exotic Unicode characters or private use Unicode codepoints).

The formula freezeFormulas finds all cells where the value begins with triple backtick, and restores them to formulas.

function freezeFormulas() {
  var range = SpreadsheetApp.getActiveSheet().getDataRange();
  var formulas = range.getFormulas();
  for (var i=0; i<formulas.length; i++) {
    for (var j=0; j<formulas[i].length; j++) {
      if (formulas[i][j]) {
        range.getCell(i+1, j+1).setValue('```'+formulas[i][j]);
      }
    }
  }
}

function unFreezeFormulas() {
  var range = SpreadsheetApp.getActiveSheet().getDataRange();
  var values = range.getValues();
  for (var i=0; i<values.length; i++) {
    for (var j=0; j<values[i].length; j++) {
      if (/^```/.test(values[i][j])) {
        range.getCell(i+1, j+1).setFormula(values[i][j].slice(3));
      }
    }
  }
}

function onOpen() {
  var menu = [{name: "Freeze Formulas", functionName: "freezeFormulas"}, {name: "Unfreeze Formulas", functionName: "unFreezeFormulas"}];
  SpreadsheetApp.getActiveSpreadsheet().addMenu("Custom", menu);
}

Short answer

Use something like ROW(A1:A21)

Explantation

ROW(cell_reference) returns the row number of the specified reference. When the reference is a range and the function is inside of a ARRAYFORMULA it will return a an array of a sequence of numbers starting on the row number corresponding to the start cell reference and ending on the row number corresponding to the end cell reference.

There are several Q&A on this site that use this.