im planning on using an around 17M adressable led strip controlled by arduino, and plan on putting it in car, so i would be stepping down the 12/14V to 5V, basically i need to know how many amps and converter/options reccomendations also would the let strip colours fade if i split it to two so lets say two 8meter strips and only put power on start of the chain of strips or do i need to run a second wire to connect somewhere in half ?
As StarCat said in a comment on your question, you are going to potentially have to supply 100 amps to the string! This would be a massive sized cable, to say nothing of the problem of finding 100 amps from your 12V battery.
You may have to size down your expectations a bit, or maybe use a LED strip that doesn't require as much as that. Your question doesn't mention any particular strip so it is hard to be specific.
Strips like the Neopixels will tend to have colours fade towards the end as the voltage drop over the string increases. They are not designed to supply tens of amps from one part of the string to the next.
You could certainly supply the power required at each end, and possibly in the middle as well, to decrease the number of amps that have to go from one end of the string to the other.
Strings like Neopixels "refresh" the data as it goes from one LED to the next, so the data part itself isn't the problem so much.