Ever wondered how to calculate the conduit size for a particular number of wires of a given size? Below are a couple of useful links. The first to an online calculator, the second to a chart so you can practise your own math skills.
This is not a be-all / end-all answer to the question. There are other factors that must be accounted for according to the NEC. For example, at a certain point the number of cables will cause a de-rating factor to be applied. This is due to heat build up from the electrical flow and non circulating air. Anyhow this is a place to start if you're wondering.
Conduit Calculator (https://edeskv2.belden.com/conduitcalculator/)
Conduit Capacity Chart (http://www.belden.com/08Tools/08_ConduitCapacityChart.cfm)
Try this link also
http://www.adc.com/productsandservices/productsolutions/truenet/conduitcalcentry.jsp