Follow the below document for the migration.
1. When we transport the object to QA, In quality also we will see these 2 transformations, one for Transfer rule and update rules,.. Is there a way where we can use only one transformation instead of two or can we merge these 2 transformations into one transformation?
If you want only one transformation, you need to create manually from datasource to infoprovider and include all the rules from two transformations.
2. Second question is in future if we want to make any changes to the transformation or data target for example adding couple of infoobejcts then where we will map those infoobjects or in which transformation we do the mapping?
If you are adding new fields to the infoprovider, you need to add them to the infosource as well so that you need to map from source to infosource and from infosource to infoprovider.