The amount of umbrella insurance coverage you will need will depend on how much assets you have that you need covered under the umbrella insurance policy.
Some people will carry an umbrella insurance policy that covers up to $1 million dollars in assets which is usually more than enough for most people.
The general guideline is that you should have more umbrella insurance coverage than you do assets so get enough umbrella insurance to cover double the amount of assets you have.
Also you will usually need to get the umbrella insurance from the same company you're getting your current homeowners insurance or auto insurance from.
Most insurance companies will only sell you an umbrella insurance policy when you have homeowners insurance or auto insurance with them.