Dawn of the Tiberium Age (DTA) is a stand-alone mod that combines Tiberian Dawn (C&C 95) and Red Alert. While classic mode gets as close to the original game-play and balance as possible, Enhanced mode provides improved game balance and a lot of fun new stuff. DTA features many customization options for Skirmish and multiplayer (where you can play as GDI, Nod, Allies and Soviet on over 200 maps), challenging original singleplayer missions, as well as co-op missions. Since this is a stand-alone mod, you don't require anything but the mod itself to be able to play: the original game is not required.
The build speed bonus given by multiple factories used to work very strangely with the Tiberian Sun engine. Building a second factory gave a very small boost of 10%, while getting a third factory then suddenly doubled your build speed. This obviously made getting a second factory useless unless you were also planning on getting a third factory later.
With the latest update dkeeton of CnCNet copied RA2's code for the multiple factory bonus, giving us more control over how the bonus works. Every extra factory now gives a cumulative build speed increase of 33%.
Does it cap at 6 factories?
Short answer: no, but I figured including the rates for 6 factories is enough for the image.
Long answer: depends on what you're building. There's a global "minimum build time" in the game that applies to every object regardless of its cost and your number of factories. Cheaper objects reach that minimum build time faster and as such get capped with fewer factories. For many (most? any?) objects it doesn't matter whether you have 10 or 20 factories. Some things most likely already cap at 6 factories, but I'm not exactly sure which ones. As an educated guess, most infantry and cheap vehicles such as buggies and humm-vees might already get capped, or near-capped.
Ah, yes, the old "Build time must not be quicker than the time it takes the previous unit to leave the factory" standard.
how about more than 6?
and wht about construction yard? nothing diferent?
More than 6 factories increase the speed further, just as I already stated above.
Construction yards are factories just like everything else, they also give the same boost.
I'd always figured the advantage of multiple CYs was the ability to expand without having to make a silo line to where you want to build (and to have a spare if you lose your primary CY). I never seen the need to speed up building times, but units absolutely.