I am stacking seven two layer, eight inch round cakes on top of each other. I am using plastic dowels hollow 3/4 inches around. and am using plastic cake boards in between every two layers of cake. How many dowel rods per tier? and should I use one board or two per tier.

Thanks any help is appreciated.

3 or 4 dowels per 'tier' (4" heighth) will be plenty. You say 'plastic cake *boards*' but are then not the cake *plates* with feet that fit into the hollow dowels?
Not sure what you mean by plastic boards.

yes disposable not the ones with feet.

and even in the very bottom tier just 3 or 4 rods?

Yeah... I'd do 4 dowels per tier.

Yes, even in the bottom tier 4 will be plenty. Those are very strong and will be more than enough.
I'm still not sure what your plastic boards are - I'm wondering if they will crack with the weight. Since I can't be sure how to advise you you might try putting a lot of weight on one board standing on 4 rods to see how it holds up.
I'm thinking plastic plate like one would use for a picnic ...... but then I'm also thinking those cake boards that are made from old milk cartons (at least that's what I was told they were made from) that look plastic but really are a coated cardboard.....
Since these are all 8" cakes there will be plenty of support - not much spread between the rods.....so one should be fine.....your dowels/rods are thick and will support plenty of weight.

I'd probably put 4 in the lowest 'tier'; then 3 in the others - staggering them so they are not over each other - understand what I mean? like maybe x in one and + in the other - but using 3.
ONe of the reasons for suggestion 3 is that you will be displacing a lot of cake with that size dowel....
propbably loosing 2 or 3 servings all together and the other is how stong they are.

ok jmo heres is what i would do in this situation, you are stacking 7 cakes that are the size on top of each other and separating each 4 in high cake with its own board. i would stick 4 rods in each cake to ensure that they hold under the pressure, then i would stick one long rod down the middle of the whole cake, boards and all to ensure that it dont come tumbling down if the "table" get bumped by accident.

Using a rod down the middle is often false security It *might* help some but don't depend on it. Making perfectly sure the cakes are perfectly level is the best security there is. Next, is using a dependable support system and those size dowels are SUPER strong.

If *I* were making this arrangement I would be using plastic drink straws in all but the lowest tier - and maybe even in it I have used them sooooo often I cannot think of not using them.

The more I think about this the more I realize this cake is going to be 14" tall (7 layers 2" each) by 8" wide. It is going to be top-heavy no matter what! It almost would be better to use one long PVC pipe (2" wide?) glued to a base board and push/thread the cakes over it - you know like we so them doing so much on those TV shows - instead of the dowels in each tier.
I hope it does not need to be transported.