In short, the more the better.
How the Claims Process Works (the Short Version)
If you want the insurance company to pay your claim, words alone are not enough - you're going to have to PROVE your injuries and damages were caused from the accident, and you'll also need to PROVE how much those injuries and damages are going to cost you.
If you approach the insurance company without any witness testimonies, crash scene police reports, doctor's bills, etc., you've got what we call a "weak case."
A winning case is one that has extensively documented the accident and its aftermath.
What Kind of Documentation Do You Need?
They say a picture's worth a thousand words, and in the case of personal injury claims this couldn't be more true. Taking photos of the accident scene, damage to the cars and your injuries are crucial to winning your injury case. Pictures can show insurance adjusters exactly how the accident happened and the damages it caused. Though memories fade away and stories can change, photos last forever.
Like memories, bruises fade over time too. Photos document what bruises, casts, cuts, strains and other disfigurements looked like right after the accident. Take pictures of all of your injuries, and continue taking pictures as they heal to give the insurance company a really clear idea of what you've been through. For example, you might be in a cast, on crutches or in a wheelchair for a period of time after your accident. That can be a miserable time in your life and photographs show others just how bad your situation was.
It's equally important to keep up with all of the paperwork (especially medical bills and notes from missed work days) relating to your claim. The paper trail will help you document how much the accident cost you. Keep copies of any notes the doctor gives you regarding missed time from work or work restriction. (There's a chance this information will not be recorded in the doctor's medical chart, but this kind of thing will be needed to legitimize any lost wages that you incur).
Find someone else who saw the accident and can testify to what happened. Witness testimonies go a long way, especially if the person was a third-party bystander.