To get an idea of pricing by #of tiers, open up a new tab/window and visit: http://www.nmartisancakes.com/wedding-and-tiered-cake-pricing.html.
The range in servings is dependent on the diameters of the tiers. Example: A 6 tier cake could be sizes 6"/8"/10"/12/"14" diameters to serve 308...or it could be 6"/9"/12"/14"/16" to serve 420. Those wider diameters on the lower tiers make a huge difference in number of servings.
You also have the option to choose just one filling flavor per tier or up to 3. Again, this is completely your call.
The bottom tiers are going to serve the most guests...so assign your favorite flavors there. Save the smaller tiers for flavors you like, but might be lower on your priority list. And if you are saving the top tier...it should be a duplicate of your most favorite option!