I have a back porch that is asphalt, I would like to put concrete over it. The asphalt is in good condition with only a few dips in it. The area is 12'x 17'. Can I use a self leveling cement, How thick will it be. Some furniture and walking only no cars will go on it.

The right thing to do would be to remove the asphalt then put in a concrete patio.

Personally I would take it up also
But yes it is done Concrete Atop Asphalt Renews Highway - 9/3/2007 - Western Builder (http://www.acppubs.com/article/CA6473424.html)

Avoid it cracking on loads it should be reinforced with 8mm mesh or rebarb. You should also use expansion joints to avoid it cracking.
Ashpalt is afelxible material Concrete on the other hand is more stiff so you need to consider expansion.

As Chuck referenced, concrete over asphalt has been used many times for road resurfacing, and with some type of reinforcement cracking should be minimal. In one case it was used as a bond breaker between old distorted concrete and new concrete to be placed. You should get some concrete material recommendations from the redi-mix supplier and go with the minimum thickness the manufacturer recommends. If you can use air entrainment with the mix you select, have it added. It adds greatly to the life of the concrete when subject to freeze-thaw cycles. With reg concrete, 5" (maybe 4" reinforced) would be about minimum. Maybe able to go lower with special mixes (manuf rec?)

Tito, I think the reason many of us would say to remove the asphalt is most patios can't take a 5" increase in height, can yours? If not when you remove the paving you should find some gravel base already in place and you may be able to pour your concrete over the old base, this isn't a roadbed.