Alan here from sunny Barnsley lol
I'm making a mould for a air intake to the air box on my Ford Focus RS its an odd shape and hopefully you could help me decide how many splits on the mould I would need ?
I need to sand down the joints then cover with filler before flatting then I plan to spray it to get a really good finish 

I would make the join somewhere along the black line.

With the sides being quite short you would probably get away with parallel sides, as long as you use plenty of release agent. Just make sure nothing converges!