If you purchase and complete the From Ashes DLC, how many new war assets will you acquire? Are there any decisions that will affect the amount you receive? Will any of your other war assets be affected?
For completing the main mission in the DLC "Priority: Eden Prime", you get the "Eden Prime Support" war asset worth 25 points.
There is a side quest (Eden Prime: Resistance Movement) in which you need to retrieve 3 pieces of intel, for each one you find you receive 25 points, doing a total of 100 points.
No other war assets will be affected by DLC.
You get 25 war assets for completing Priority: Eden Prime and another 25 for each of the three pieces of intel on the Eden Prime: Resistance Movement mission. So that makes 100 in total. You can't lose any of your existing war assets.
The DLC have some impact on the war asset but only during the mission.
Indeed you'll find on the Cerberus camp, data that will help you support Eden Prime resistances forces against Cerberus. Each of them found will provide you 25 war assets up to 100 which will appear undear the name of "Eden Prime Support" in the Aliiance War Asset.
Except that any decision made will have an impact on war assets.
Source: Personnal experience and IGN Wiki