I am considering buying an electric car each year to take advantage of the tax credit but I am not sure if it's a once per lifetime credit?
Yes, if you purchase the eligible electric vehicle during the tax year you can use the credit for that tax year. If the credit reduces your tax liability to zero any further credit is lost. This is not a once in a lifetime credit.
See the IRS instructions for Form 8936 Qualified Plug-in Electric Drive Motor Vehicle Credit - https://www.irs.gov/instructions/i8936