I usually use 3 and haven't had a problem so far. I think you should be fine with 5. I drove about 45 minutes to a venue as well and didn't have an issue at all.
I used 3 cakeboard for my first 3 tier that was 7", 8.5", 10" and it traveled fine for about 45 mins, so 5 should be ok for your size cake.
I was given great advice in another post by dkelly who recommended alternating the currogated lines of the cakeboard when you glue them together to reinforce its strength.
HTH. 
I think you will be fine with the boards, but it never hurts to be extra cautious since it will probably set your mind at ease a little. If you still have doubts & don't mind using a square base, you could always go to Home Depot or Lowes & they can cut a piece of 1/2" plywood for you in the size you need. Then you could cover it before placing the cake on it. You can also find square tiles there to use as well.
Love using the tiles....even have pretty designs on them so you may not even have to 'cover' them! Have a wedding coming up that I found just the right size/color my last 'cake shopping' trip to Home Depot! I do like to put a complimenting thin ribbon around the edge using a heavy duty glue stick or glue gun.
I think you will be fine with the boards, but it never hurts to be extra cautious since it will probably set your mind at ease a little.
If you still have doubts & don't mind using a square base, you could always go to Home Depot or Lowes & they can cut a piece of 1/2" plywood for you in the size you need. Then you could cover it before placing the cake on it.
This is what i use for all my large cakes - then i buy little clear door stops that you stick on the wall and stick them to the bottom of my board - that way you get anout 1/4 inch gap so as to get your fingers underneath to lift.
If covering a cake in fondant/regalice - i just cover the board in that and decorate it .
You can also find square tiles there to use as well.
Thankyou for typing that - i will be hunting for a lovely large red/black/white tile for next weekend to sit a birthday cake on
Brillante Idea - thanks again
Bluehue.
I just started using the square tiles and LOVE them. Don't have to worry about the cake cracking when it's getting carried in. LOTS of good support and CHEAP. One thing I do though is I cut a piece of the non skid rubber to size and hot glue it to the bottoom of the board so that it doesn't scratch up someones table or counter top.
I was at Lowe's a couple of weeks ago trying to figure out what kind of wood to have cut for a 16, 12, 8" square wedding cake. I looked at tiles - they had some on clearance - and thought they would be fun, but the biggest I saw was 12x12. Will you CC'ers who use tiles tell me what you do with them?
Thanks!
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