Hi Mark,
Sorry for the delayed response.
Thank you for your interest in Syncfusion products.
Yes, it is possible to limit the number of columns to be merged in the document using Essential DocIO. I have attached a sample for your reference. Where the sample makes use of a DataTable as DataSource. The Number of columns the DataTable is 8. I have made use of only 3 columns for merging.
Here is the sample for your reference.
MailMerge.zipPlease check it out and let me if you face any issues.
Regards,
Sarath