How does the capacity of 2 parallel runways compare to single runway?
Definition
There are two key performance indicators that are used to measure the key performance area 'airport capacity':

Peak arrival [declared] capacity
Peak arrival throughput.



What is the capacity of 2 near parallel runways?

It depends if the 2 runways are operated in mixed or segregated mode.

For the US, the two active runway case average value (47) is influenced by the ability to operate in mixed mode with independent runways for Tampa (TPA) and Portland (PDX). Otherwise the grouping is more comparable.

The average is between 40-47 arrivals per hour depending on the mode of operation, the higher value being influenced by the mixed mode operation.

How does it compare to mixed mode single runway?

Single runway is 29-30 arrivals per hour. So we are looking at a 33-50% increase.
Declared and IFR
I'd be careful with the interpretation (since you tagged it IFR), because one of the reasons you see a larger gap between the declared and actual for the US compared to Europe is because in the US the capacity is typically declared based on visual meteorological conditions. This is discussed here:

Why are visual approaches (in the US) preferred by US pilots, but not European pilots?
What are the practical impacts of airports in the US being mostly IATA Level 2?


Source of graph and quotations: Comparison of Air Traffic Management-Related Operational Performance: U.S./Europe, 2015, eurocontrol.int (.pdf inside .zip).