It's complicated. There were about 2-3x as many people on the trans-supporting ones, but they were organized by the minority parties in the governing coalition, while the majority parties in the governing coalition were on the no-trans ones. Meanwhile, almost 50% of the population voted right-wing and populist extreme-right, who also organized some marches with about 1/10th the people. So it's hard to say which side has actually more support. It also wasn't a vote, so some of the same people could have been to more than one march 🤷

If you come to Barcelona in the following year or so, you'll also have to deal with the Catalonian nationalist separatists, who are an orthogonal split to everything else, and right now it isn't really clear what they want (not to be part of Spain, not to be ruled by the King, but stay part of the EU, maybe just get a separate representation in the EU, but the EU is pushing towards a federal structure... it's also complicated).