The 12-month policy will generally start on the date that the last car insurance is up for renewal, with short-term cover used to protect the other cars in the interim. For example, if you have one car that needs to be insured from February and another in June, the February car will be insured first but the policy will actually renew in June when both cars are covered. After the last car has been insured for a full 12 months, the policy will renew each year until you choose another policy.
Auto insurance is financial protection, and not just for the investment you made when you bought your car. After a really serious accident, bills for damage and injuries can easily reach into hundreds of thousands of dollars. If you happen to cause such a wreck, the victims could sue you. In the worst case scenario, assets such as your savings and home could be seized.