“Fair use” is probably not the correct term as it is about rights in content, whereas what you’re asking is “how much load can I put on OSM’s servers” and the following policy explains that a little (while not giving concrete numbers): Tile Usage Policy

The web site https://switch2osm.org has instructions on how to set up your own tile server. You could also investigate making a static image of the map with the marker and host that on your platform - of course this doesn’t give the user the option to zoom and pan, but you’d not make any tile requests from OSM when the page is served.

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