There are a few of issues here.
1. The cook time will be similar whether you are cooking 1 Tri-Tip or 5. Just make sure you leave a few inches between the pieces of meat. If they are too close together, they will cook like a larger roast.
2. NEVER trust the time and temperature information given in a recipe. The information is to help you plan your cook. ALWAYS verify the internal temperature of each piece of meat using an instant read thermometer you know to be accurate. When cooking beef, a difference of just a few degrees of internal temperature can make a difference between medium rare and medium well. If you prefer medium rare and end up with medium well, you won't be happy.
There are too many variable in outdoor cooking to be able to cook to time and temperature as you might do in a kitchen oven. But even indoors, using a meat thermometer is recommended.
3. Always allow extra time after the cook is completed for the meat to rest. That allows juices to redistribute throughout the roast. Somewhere betweeen 15-30 minutes would be a nice nap for your tri-tips.