Numbers are easy, until marketing and/or legal gets involved.
Porsche numbered their cars based on project number. So the iconic 356 is the 356th project that Porsche Engineering undertook.
Except Ferry didn't want his first customer to think they were the first... so the first project was #7, so the 356 is actually project number 349 (this is where marketing kicked in for him).
This carried on with sub assemblies - the 744 transmission, etc.
Then the 901 was introduced. And after they made 34 cars, Peugeot called their legal department and it was decided that they had an issue with any other car maker making a car and badging wtih a 3 digit number where the middle number is a 0. And so overnight the 901 became the 911.... of course, one of those first 34 cars with the 901 badging are VERY collectible, even over and above any other early 911...