Parker Solar Probe is designed to go as close as 10 solar radii from the Sun (1 solar radii = 695,510 km). On its first 3 orbits, Probe passed within 35 solar radii. In late December, Probe will use a Venus flyby to reduce the closest approach distance to about 28 solar radii. Probe will pass within 10 solar radii on its 22 orbits in December 2024. That's as far as the mission is planned by NASA as of now. It uses a big heat shield to protect the spacecraft and instruments from the intense solar radiation. There is probably margin in the design of the heat shield, so who knows? By 2024, they may decide to try to go closer and see if Probe can survive!