I did this in a rush. See if it works for you.
Open a new document before running the script. Units are in points.
var ws = [], hs = [], n;
var dialog1 = new Window("dialog");
dialog1.add("statictext", undefined, "How many artboards?");
var input1 = dialog1.add("edittext", undefined, "n");
input1.characters = 4;
input1.active = true;
input1.onChange = function () {n = Number(input1.text);};
var button1 = dialog1.add("button", undefined, "Next");
button1.onClick = function () {
var dialog2 = new Window("dialog");
for (var i = 0; i < Math.min(n, 20); i++) {
(function() {
var group = dialog2.add("group");
group.add("statictext", undefined, "#" + (i + 1));
var w = group.add("edittext", undefined, "w");
w.characters = 4;
w.onChange = function () {ws.push(w.text);}
group.add("statictext", undefined, "x");
var h = group.add("edittext", undefined, "h");
h.characters = 4;
h.onChange = function () {hs.push(h.text);}
} ());
}
var button2 = dialog2.add("button", undefined, "OK");
dialog2.show();
dialog1.close();
};
dialog1.show();
var doc = app.activeDocument;
var AR = doc.artboards[0].artboardRect = [0, 0, ws[0], - hs[0]];
for (var i = 1; i < n; i++) {
doc.artboards.add([AR[0], AR[3] - 10, ws[i], (AR[3] - 10 - hs[i])]);
AR = doc.artboards[i].artboardRect;
}