one reducer will run ,it has nothing to do with part of file name , no of reducer would be either specified by the user by default it calculated the size of the input file and amount of work which need to be done in reducers .

part-r-00000 : This is related with partitioning, Since we have one reducer so all partitions will point to this file 

Number of reduces in most cases specified by users. It mostly depends on amount of work, which need to be done in reducers. But their number should not be very big, because of algorithm, used by Mapper to distribute data among reducers. Some frameworks, like Hive can calculate number of reducers using empirical 1GB output per reducer.

1.这个输出突出了multipleoutputs和multipleoutputformat之间的一个关键区别。当使用multipleoutputs时，您可以输出到reducer的常规outputcollector，或输出到指定输出的outputcollector，或同时输出到这两者，这就是为什么您会看到part-nnnnn文件。
2.根据不同减速机数量的对标，可以确定最佳的减速机数量。这也取决于数据的处理。
例如，我们将使用2个减缩器处理1gb，这会产生一小部分处理。但是对于其他mr作业，我们需要为1gb文件设置5个减缩器，这会产生某种巨大的处理/计算。所以最好设置基准。