If you're going to multi-table on a single screen (laptop or not), then you're going to have to stack after about 4-6 tables, which is about the most you can tile and still be readable. Maybe less on cheaper, low-resolution laptops. When you stack, it doesn't matter how small your screen is or whether it's a laptop or not, because the tables are on top of each other and you're only ever seeing one whole table at a time.
Whether you have a mouse or not is also up to you and not really relevant to how many tables can be played. There's nothing about
online poker that requires an external mouse. Some people prefer the mouse, others the trackpad. If you already have a preference for non-poker work, use that.