The amount needed depends on how much you make, how much you need to live on and other factors.. Yes, you should get disability insurance. First place to look is what is offered through your employer. Take advantage of that and elect the maximum coverage they offer. Find out whether that is a pre or post tax deduction. Then work with a professional to find the "gap" in your income which will dictate how much you need for your own personal policy.
The best way to think about this is that you’re trying to replace your net income in the unfortunate case in which you become disabled.
There are two types of disability policies. Short term that covers you up to 6 months, and a long term one that can be until you reach full retirement age
Everything that Julian said previously is spot on with regards to the route that you should take with regards to disability insurance. What you might run into is that your employer may or may not provide group disability insurance depending on its size. Another thing that you need to pay attention to is that there are two types of disability insurance specifically short-term and long-term. Short-term disability insurance goes anywhere from 7 to 90 days depending on the policy that your employer might provide to you. Long-term disability insurance is anything over 90 days to a preset time period within the policy. More then likely, your employer's group policy if provided only has short-term disability insurance and thus you would have to supplement it with an additional short-term disability policy to cover the income "gap" and you would have to purchase a long-term disability insurance policy. When I refer to "gap", disability insurance will only cover to a maximum roughly 60-67% of your post-tax income. You will never make as much as you did by working and this intentional by the insurance companies. But overall, disability insurance, employer and individual, is an underutilized insurance in most instances by individuals. You dont think about it until you need it the most.