Is it legal for my insurance company to total my car if I’ve only got one estimate?

Your insurance policy dictates what percentage of damage vs the value of the car; You can ask that they add any recent improvements to the vehicles value which is not considered normal maintenance; Also if you want to keep the car and fix it yourself, you can accept their amount minus what the salvage fee for the car; usually 100-200 depending on what they want to sell it back to you; You can then fix the car and drive it, it will however have a salvaged title and only will be insured for liability if they want to do that