Most Neutral Density filters are available in two, three or four stops† (below) but, if this isn't enough of an exposure increase and doesn't create the effect you want, you can combine multiple filters for extra strength.
There are also a couple of 10-stop filters on the market, including Lee Filters's new Big Stopper (below, centre), a 100mm by 100mm square filter that fits into the regular Lee holder, enabling it to be used in conjunction with other filters such as Grads. B+W's 10-stop filter is a circular screw-on filter, which is a drawback if you want to add other filters too, such as Grads.
Both the Big Stopper and B+W filter have a slight colour cast due to the material they're made from and the way that they transmit certain wavelengths of colour, and so are not truly neutral. However, one click of the auto white balance button in raw processing will bring them back to neutral. Lee Filters Grads also come with hard or soft edges (above, left & right). The soft ones are useful for uneven horizons.