As Martijn already stated, you can create the sequence number in a microflow. However, since you do not want this sequence number as an attribute in your entity, you may want to use non persistent entities which are created in this microflow, showing the data you want to present in the datagrid, and in addition to the information from your persistent entity also comprise a field “Sequence”.
You could fill the field with a microflow looping through all the records using an iterator. But why do you only want to display the sequence number on your data grid?
I've used CSS in the past, but it was on a listview. You might get it to work on your datagrid?
For more details check https://css-tricks.com/custom-list-number-styling/