If two people start a company are they founders or co-founders?

Is the word founder used only if there is one person starting a company? If you have two or more, are you automatically labeled as co-founders?

I see a lot of people use co-founder when there are more than one starting team members, but the founders of our country weren't called the "Founding Co-Founders".

Also, if you type and/or say the word founder ten times, it starts to lose all meaning.