I need a cake to serve 30 people if I did a deep 8" round cake say 6" deep would it be enough?

The problem w/a 6" tall cake is thata serving does not fit on most plates made for 'dessert' :(   And you do not get more servings out of it - unless you use a cake board between two 3" tall layers.  Then, you each section will serve 25 pieces - each one 1x2x3 so a total of 50 servings just a tinly bit smaller than what is considered the 'industry standard' serving. 

Ok so a 10" would be better then?

Yep, a 9 or 10 - either one 3 or 4" tall should do just fine.  See, that's a number just inbetween cake pan sizes.  If I remember right a 9x4 serves 35 or 39.   I can't think of any round size that yields just 30.  Now, a 8"x4" sq should serve 32 - that's probably the closest. 

Thank you so much x