Hello @abihoughton27 ,
Yup, I do get it now, but again you will need a column to uniquely identify these rows because the Flow triggers for an item where any kind of modification is done to any column. To selectively run for specific items, you need to filter the items out based on the values in the column.
Maybe add a new column "SendEmail" and update the value to Yes for the 30 items in question the rest will remain blank.
Then in the trigger condition of your Flow check for this field to be equal to Yes, this ensures that the items where SendEmail is Yes, the Flow will trigger irrespective of which column is modified. But as a conclusion, you will need a column to filter the items on which the Flow needs to run.
I hope this makes sense and answers your question