This example uses Count, together with a Simple Math module, to truncate a feed to half its original size.
First, we pipe our feed into a Split module. This gives us two identical feeds to work with. One feed is piped into Count, which outputs a number: the number of items in the feed. We pipe this value into a Simple Math module and divide it in half.
The other feed (after the split) is piped into a Truncate module, which allows us to limit the number of items in the output feed. Truncate requires a number value (the number of items you want output), so we just wire the Simple Math output into Truncate's numeric field, and we've cut the output feed in half.
Pipe Web Address: http://pipes.yahoo.com/pipes_team/Clk7IuFS3BGj08gh8jxBKg