Depends on the competitiveness/saturation of the region(s) applying, whether or not you’re “known” to the site/networking via training directors/programs, how many positions are offered at each site, etc.
I’d recommend 10-15 apps if from a super-saturated area and no connections plus high competitiveness of positions and limiting your area of search. If you have connections or apply to less competitive/saturated areas or cast a wider net geographically to capture some rural sites, maybe 5-10. It’s a wide range, but depends on individual circumstances. Connections are more important at this stage if you’re in a popular track, from what I experienced and saw amongst colleagues.