Smoking today - first time

I've got a Brinkmann vertical smoker (Trailmaster Limited Edition) and will attempt to smoke some tri-tip roast today.  My first time.  I've got four tri-tips totaling maybe 12 pounds.  The recipes I'm pulling up all seem to call for about 2 hours of cook time for a single 2-3 pound tri-tip roast.  My question is, should I add additional cook time due to the larger amount of meat I'm smoking?  How long should I plan on cooking the meat?  And I may as well ask what the optimum cooking temperature should be in this case.  

Thanks for any help!