I did a 9+ hour flight followed by a long car journey when my eldest son was 7 months old.
He was only having 4 bottles in a 24 hour period, so technically i could have gotten away with taking 4 pre-washed/pre-sterilized bottles and only 4 cans of pre-made formula, but i doubled everything just incase and to allow for any delays/problems.
I didn't want to do hand washing and sterilizing on a plane as who knows how clean everything is

.
But i'm glad i took so much stuff as i did have problems coming back from Heathrow to Vancouver. Coming through security (in Heathrow) they decided that they wanted me to taste test every single SEALED store-bought can of pre-made formula. I explained that once a can is opened, it only has 1 hour lifetime or it has to be tossed away (obviously not going to work out on a 9+ hour flight!). They told me that it was tough, i would have to buy more once through security. I explained that i don't live in the UK anymore and my son is on very different formula to what they have in the UK. Again, i was told that it was tough. I fought it out for about an hour with every possible person in security and eventually got away with tasting 1 or 2 cans, and thank god i did because i was on a redeye and every single store in the airport was closed (i didn't even know they closed in airports lol) so my son would have starved

!!
Just be safe and take extra of everything!!!