A UPF 25 jersey blocks about 96% of UV radiation, which is solid protection for most everyday riding. If you're doing typical road rides, commutes, or weekend spins with moderate sun exposure, UPF 25 is generally plenty. You'll get good coverage without the slight breathability trade-off that some UPF 50+ fabrics can have.
That said, UPF 25 isn't the best pick if you have very fair or sun-sensitive skin, or if you regularly ride at higher altitudes where UV exposure ramps up significantly. In those cases, UPF 50+ is worth the upgrade. Same goes for long summer centuries or multi-day tours where you're in the saddle for hours on end.
One thing to keep in mind: even with a UPF 25 jersey, don't skip sunscreen on exposed areas like your neck, arms, and the back of your hands. And if your jersey fits snugly and stretches over your shoulders, UV can penetrate more easily through the stretched fabric, so extra sunscreen there isn't a bad idea.