Various licensing does not attach to a set dollar figure for insurance coverage. That will be completely determined by your comfort level and what type of clients you want to land. Naturally, the more exposure (assets) you have, the more insurance you should carry. Plus, there is the moral issue of carrying enough insurance to compensate a person if an accident happens.
Separating your business from your personal assets by filing to become an LLC or an Inc., is also a good idea (according to my CPA's advice & others) - whatever suits your situation best.
The only limitations you might experience would be from potential clients, say an apartment complex or a real estate agency that could require you to carry miminum liability coverage of "X" amount of dollars. The 1M seems to be a pretty popular figure among the lawnsite members, but I've seen some guys with larger companies posting that they carry 5M and more. You might be able to get by with less, say 300K - 500K and still land some commercial jobs, but I say why take a chance? If at all possible go ahead and spend the couple of hundred extra bucks to get the 1M policy.