How many bags of maize should a farmer expect to harvest per acre after planting PAN variety of maize seed?

The answer is 30 bags/acre if you use zai pits (according to an article by John Njue Njiru on http://johnnjue.blogspot.com/2012/05/...)

Zai pits are pits are planting pits with a diameter of 20-40 cm and a depth of 10-20 cm, with application of manure, where up to nine maize seed are planted. The pit conserves moisture for the crop and prevents soil erosion. The dimensions vary according to the type of soil dug
during the dry season it retains water for a long time . It has been used to grow bananas, sorghum, maize and millets successfully. You can get 9 maize plants in a pit

I have not see where John Njue Njiru got his information. But it is important to realize that it is likely that there is no definite answer and the yield depends on the work invested. Yatta is a dry region as you know. Recently agriculture was very underdeveloped. Some new techniques of creating water pans have been very helpful. One major example is the "Operation Relief Food Out” introduced by Bishop Dr Titus Masika of Christian Impact Mission Church in partnership with the USAID and the Kenya Horticultural Competitiveness Project (KHCP). -
see this video https://www.youtube.com/watch?v=SEQQU...
And their webpage http://www.christianimpactmission.org...

They encourage the planting of trees and the development of water pans for irrigation. If this is done then farms can be very productive without having harvests tied to the rains.
Here is a story of one such farmer here: http://liliankaivilu.com/4208/can-uka...

Do you have an irrigation system?
Do you have fertilizer?
Are you currently growing maize
Why do you want to grow maize? To eat it or sell it?

The yields of maize in Yatta District can be categorized in to two scenarios because of the variations in rainfall patterns in the region.
Scenario 1. When rains are adequate
Scenario 2. When rains are significantly below average

Average yield per acre per household
Scenario one - varies between 10 bags to 15 bags (90-Kg bags)
Scenario two - varies between 2 bags - to 5 bags (90 - Kg bags)

Variable costs per acre per household
Scenario one - 7,850 KShs
Scenario two - 4, 290 KShs

Gross Margin per acre per household
Scenario one - 5,647 KShs
Scenario two - 210 KShs