What size and how many tiers would I need to feed 150 people for a wedding?

This is my first wedding cake so I am not sure about the servings etc (lol.. see my other post)

what other post?

Here is the wilton wedding cake serving chart. Most venues cut by this chart: http://www.wilton.com/wedding/wedding-cakes/wedding-cake-data.cfm

Here's a link to how to cut a cake to achieve those servings: http://www.cateritsimple.com/id10.html

Here's cut pieces of 1x2x4" pieces of cake to show you that they are not "paper thin":
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1156785
and
http://www.flickr.com/photos/55969028@N00/3856884667/

What shape cakes are you planning to use? Different size tiers are needed depending on whether you are doing round, square, hexagonal, etc shaped cake.

I don't know yet. . the bride hasn't specified anything. Square or Round. . or possibly a combination of both.

I use Earlene's serving chart
http://www.earlenescakes.com/ckserchart.htm