Typically one heavy coat of primer is sufficient for most exterior painting projects. Sometimes two coats of primer should be applied if the surface is still porous. This often occurs when trying to paint very porous wood like cedar or T1-11 siding.
Another example is priming over old and cracked oil based paint. With this situation I use a penetrating latex sealer/primer, such as Peel Stop, before applying an oil based primer. The sealer/primer helps to glue down the paint edges while the oil based primer stabilizes the surface and readies it for painting.
Start with a good heavy coat of primer and apply another if you feel it is necessary.