You can never have enough insurance. The limitation is how much you can afford to pay. Most insurance companies have a minimum premium, regardless of how small your risk is. That’s why insuring your first billboard is the most expensive. You can normally add additional billboards on for a nominal amount.
I would seek $1,000,000 in liability insurance, and then a balloon (if possible) of another $2,000,000 or so. I think that $3,000,000 will cover you in all but the worst-case scenarios. It’s certainly enough money to make a personal injury attorney want to settle, as opposed to going to court. If I had to give on something, it would probably be the insuring of the structure itself. They rarely come down and, if so, it may be an “act of God” and not covered under your policy – make sure that you know what exactly is covered.
Be sure to get as many bids as you can. I would try and get as many as 10 quotes to compare. It is amazing how different the quotes can be. I would go with the cheapest one that is written on a decent credit quality company. Tell the company that you need to know what it will cost to insure maybe 10 similar signs under the policy – so you know what the incremental insurance cost is of each new sign added, so you can budget accordingly.
One super important point – never go without insurance. If you can’t afford decent insurance for the sign, then flip the lease to another company as opposed to operating without insurance. I have seen some terrible claims over time, and there is no way that it is worth the gamble.