How many days is "a few days" in Seville? There's a lot to do there.
Ronda's quite far from Seville for a day-trip. Renfe's website (https://www.renfe.com/es/en shows a travel time of over 3 hours each way.
Damas runs a few buses a day between Seville's Plaza de Armas Bus Station and Ronda. The fastest takes 2 hr. 15 min. A bus would probably be considerably cheaper than a train. By train, you are likely to be following a roundabout route that takes advantage of the high-speed rail lines. You pay for all the extra mileage.
At the time of my 2019 trip to Andalucia, there was at least one company offering a bus tour from Seville that included both Ronda and one or more of the white villages. That would be a rushed visit to Ronda, which does have several points of interest. I don't know whether there are tours dedicated solely to Ronda; they'd certainly cost more than a do-it-yourself- bus trip.
You are likely to save money on express-train tickets by buying in advance. Note the different cancellation/change options offered if you decide to buy early. Some bus fares can vary, as well, but I think the changes are not as extreme as those you sometimes encounter with train tickets. Trains between Seville and Cordoba (and on up the line to Madrid and Barcelona) definitely can sell out. That's not a rare occurrence in Spain.
I've seen buses sell out as well. I don't think that happens as often, or very far ahead of the departure date.
If you happen to be traveling in Andalucia during Holy Week, I'd be especially concerned about sold-out inter-city transportation.
Are you going to Cordoba? I believe nearly everyone would recommend Cordoba over Ronda, and the side trip by train to Ronda would be faster (and I assume cheaper) from Cordoba than from Seville; the express-train route from Seville to Ronda runs through Cordoba (never mind what the map tells you).