Travel from Barcelona to Paris

Eurail is a pass you can buy to ride trains in Europe. It is not a train operator. Here is information about the train from Barcelona to Paris.
https://www.seat61.com/international-trains/trains-from-Barcelona.htm#Spain-Paris

You can check the stops easily at https://int.bahn.de/en.
The main station in Barcelona is Sants, and it arrives at Gare de Lyon in Paris. Click on "stops" after "details" to see where it stops.

Since each leg requires a seat reservation, not very convenient for hop-off/on. There are only 2 direct trains every day

You would need to buy separate tickets for each leg if you were to get off. For example, Barcelona to Nîmes and then Nîmes to Paris.

To say that hop on hop off is hard and there are only two trains a day which require reservations is very wide of the mark.
The first two stops in Spain are pick up only so you would visit those using the frequent local trains. All other stops in France have quite frequent TER local services linking them- fixed fares, no reservations, walk up and buy.
From the final stop, Valence, there is a frequent TGV service to Paris.
But even then you can take TER trains to Lyon for the TGV to Paris.

Taking local trains you arrive at the city centre stations, not the out of town high speed stations which several of the direct high speed trains use.
So in summary the hop on hop off approach can be done with ease.

Just a couple of corrections on the above post: all the stops in France for the train from Barcelona are at center city TGV stations, one of the reasons I used Nîmes as an example. Montpellier or Narbonne also would be within a loosely defined halfway mark timewise.
Truthfully, a person could take local trains all the way from Barcelona to Paris if he or she had a couple of days and a lot of patience, bucket loads of patience.
Finally, isn31c is correct that there are more than two trains: SNCF has 3 direct trains daily. As for Renfe, I don't know.