How much area does a 1/4" drip tube(not soaker hose) saturate on either side of the tube?

I'm trying to plan for how much tubing and other supplies I will need before I purchase and can't really find the answer online anywhere.

I have an area of 8'x16' and am planning 3 rows that are 2' wide by 15' long. I need to know if one 15' drip tube running down the middle of each row is enough or do I need 2 in each row?

Additional info: there will be no emitters spraying water, just holes every few inches. The tubes will be placed on the dirt but under mulch. We are trying to prevent as much water loss(or encourage water retention?) as possible in our arid area.

Any help from those experienced in drip irrigation will be greatly appreciated!

Thanks so much.