Sirloin Tip Kebab cooking times
There is no way for us to give you absolute cooking times. There are too many variables, among them:


How do you like your meat cooked?
How hot is your fire?
How far are the kabobs from the fire (since kabob cooking is essentially 100% radiation heating, distance is a huge factor)
What is the size of your meat chunks?
What is the starting temperature of your meat chunks?


You will note that the number of kabobs is not a factor.

Instead, you need to learn to check for when they are done to your liking. This will come from experience.  Kabobs are generally too small to use a thermometer on; you would need a very high quality one like a Thermapen in order to get a reasonable reading.

All that said, it is likely that you want something on the order of 2-3 minutes per side, but there are far too many factors to make that a concrete recommendation. 

You can start by cooking just one kabob, or even one chunk.   When you think it looks good, try it.  Of it is overcooked, do the next one for less time; undercooked, cook a little longer.  After a couple, you will know about how long you will need for your particular set of circumstances.