Separate Risk Assessment

I'm assuming that at the moment only your head office will be ISO 27001 certified, and the other units will work to fulfill ISO 27001 requirements, not seeking certification at this moment.

Considering that, a good approach would be to split the branches into groups with similar characteristics (e.g., size, location, etc.), so you can identify general solutions for them and let them work on their specifics. ISO 27001 does not require a separate risk assessment for each branch.
 
For example, considering the asset inventory, you can have a single general inventory covering as many branches as possible, and specific inventories according to the needs of each branch. In the case of risk assessment, you can start with a general risk assessment covering all branches, and include specific risks for specific branches as needed.

In situations like this, the best approach is "start global, specify local", i.e., define as many general solutions as possible, allowing each branch to customize them according to their needs (based on results of risk assessment and applicable legal requirements).  

These articles will provide you a further explanation about risk assessment and inventory:

• ISO 27001/ISO 27005 risk assessment & treatment – 6 basic steps https://advisera.com/27001academy/knowledgebase/iso-27001-risk-assessment-treatment-6-basic-steps/
• How to handle Asset register (Asset inventory) according to ISO 27001 https://advisera.com/27001academy/knowledgebase/how-to-handle-asset-register-asset-inventory-according-to-iso-27001/

Thank you, @Rhand Leal