What is the usual span for two 2x8 nailed side by side. Im building a new deck 24' x 24' I don't know if I need this much support (2x8) nailed side by sdie

You really need to know that load you will carry and most cities will spell out what beams are needed. There is absolutely no way you can span 24 ' with two 2x8s so you must have some addition supports planned inside the perimeter of the deck. How far apart will these beam supports be from each other? Also, you can increase the load capacity of two 2x8s by sandwiching in a piece of 1/2' plywood, glued and nailed.

I'm assuming that your actual rafter/joist span from the ledger (house-side support) to the header beam (yard-side support) will be 22'. That assumes a 2' eave beyond the outside edge of the header to total the 24' projection. I don't know where you are in the world, but the "standard" limits for a "live-load" deck vary depending on the seismic, wind and snow loads that are established in your location and a 22' span is unusually long. I would imagine that a span of 22' for a deck will probably require a structural or civil engineer's calculations and design specifications be provided with your building permit proposal (drawings)and permit application.

Here in the Southern California desert (LA County) something like that would either require some kind of "mid-span" support (like another post/header beam) or steel "I" beams as rafter/joists. The longest span that I've ever used wooden joists for was 18' and the engineering required a 6X12 header beam and 4X10 joists. It was way heavy. Usually an engineer (whose is exposed to the structural liability of the project), will over-engineer the design to "cover his/her a--".

It doesn't cost anything to talk to your local building department folks... get some feed-back from them first. Good luck.