Three questions
Who requires a limit of you, and what is it? Who could you injure or whose property could you damage, and how badly? What would the next million cost? The first answer sets the floor; the second argues for more; the third tells you whether the extra is cheap relative to the exposure.
The role of the umbrella
Rather than raising general liability alone, an umbrella adds the same extra limit across general liability, auto and employers' liability. For a business with vehicles it is usually the better way to meet a $5M requirement.
In the wallet
Your contract requirements are matched against your policies, so a limit that falls short of a lease or client agreement shows as a gap with the figures. Quotes for a higher limit start from the application you already completed.