In series or in parallel? Using PWM? If so what frequency? Higher speeds of PWM would really need a MOSFET driver chip to get fast switching.If you want to drive the MOSFET directly from an Arduino you _must_ get a logic-level FET.What maximum current are you needing and what's the power supply(see my first question)? You need to calculatethe power dissipation in the FET using I-squared-R (ignore the nominal current rating of a MOSFET,its only applicable if using extreme cooling measures).
OK, so you need a very low Rds(on) device - heat dissipation is I-squared-R, so if the device were 10 milliohm thatwould be 8W dissipated when on, which would need a reasonable amount of cooling (something like a chipsetheatsink and fan perhaps?). If you got a 3 milliohm device you'd run a lot cooler or need less heatsinking.I've used an IRLS3036 in the past, its logic level, has 60V rating and 2 milliohm. However its surface-mount only.I'd recommend trying to get a device with Vds voltage rating of 20V or more, about 5 milliohm Rds(on) or thereaboutsand logic-level - TO220 package is probably easiest to heatsink (remember the drain terminal is also connected electricallyto the tab, so you need an insulating heat-conducting pad between package and heat-sink.Another approach is to parallel a couple of devices to get half the Rds(on) figure - then 10 to 15 milliohm devices would be more feasible.