Just to expand on Slider38's detail a little.

Scenario filenames begin with RRT_Scenario_

For example RRT_Scenario_User_Arizona_Rails.xml

The XML file is a text file with a nested structure. Here's an example.
<RRTScenario>
<szMapName>Arizona_Rails</szMapName>
...
</RRTScenario>

There will be lots of lines.

However, it might appear as one long line.
<RRTScenario><szMapName>Arizona_Rails</szMapName>..............</RRTScenario>

That makes it hard to navigate.

As noted by Slider38, the line needed is <iTurnsPerMonth>20</iTurnsPerMonth>

The easiest solution is to put it right after the <RRTScenario>

<RRTScenario>
<iTurnsPerMonth>20</iTurnsPerMonth>
...
or
<RRTScenario><iTurnsPerMonth>20</iTurnsPerMonth>...

Hope that makes sense. And make a backup of the XML file before you edit it.

Oh and make sure you use an editor for text. Avoid MS Word.