Click the "Metronome Setup" icon (small wrench/tool) above the word "Metronome" on the transport panel at the bottom of the screen. Then in the window that pops up, change the "[ 1 ] Bars" to how many bars you'd like. This is always based on your song's time signature, so if you have a 4/4 song with 2 bars precount, it will precount 8 beats, and so on.