About ~5x improvement. Recall that the resolution of an interferometric array is set by the distance between telescopes measured in units of the observation wavelength. BHEX will get a ~3x resolution improvement from the increased distance between the space satellite and our ground telescopes (for the EHT, the max telescope separation is limited by the diameter of the Earth) and another ~50% from the increased frequency of observations (going up from 230 to 320 GHz).

5x is actually a lot: we'll be able to resolve the "photon ring" of orbiting light around M87* and Sgr A* (the two black holes previously imaged by EHT at lower resolution) and likely see the "shadows" of another 6-8 black holes, with the possibility of estimating the mass of another ~20-30 sources.