Zero.
In fact, you need to get under there and hold it up right now with your hands as it's going to fall down in just a second!
Not enough information.
Are these ceiling joists or roof rafters for a flat roof?
Are you asking if the roof will support the required loads for your area, or are you asking how much weight a storage area above the ceiling will support?
What is the spacing of the 2x6's?
What are the 2x6's made of:
Is the ceiling going to be insulated?
What is the ceiling material going to be?
What else is going to be hung from the structure?
All these questions need answers to find out what you need.
For example, using these ceiling joist and rafter tables, for a 30# live load and a 10# dead load, you would need 2x12's at 16" on center to support it if you wanted a drywall ceiling which requires a maximum deflection of 1/360th of the span. This is using Hem-fir with an E of 1.3
http://www.awc.org/pdf/STJR_2005.pdf
Live load is the weight you can put on in pounds per square foot, evenly distributed. Dead load is the weight of the structure, including the joists, decking, insulation, ceiling and anything suspended from the ceiling, like lights.
Even Hem-fir 2x6's at 12" on center, using l/180 (Which means you can't use a drywall ceiling) and based on a 20#/SF live load and a 10#/SF dead load will only span 14'9".
What you have will keep the walls from bowing out, but little else, based on typical materials. A stronger material, like Doug-fir will help some, but the real solution is deeper joists.
All this said, if you don't care about deflection, aren't putting in a drywall ceiling, just have a few lights, and are storing light things near the edges of the span (Closer to the walls), you will be OK.
If you are asking about the rafters, and their ability to support the required live and possibly snow loads for your area, then we need to know what those required load capabilities are. Where are you, and what are those loads?
Show us some photos
