Can Mapnik render a symbol at every junction?

This is not a question of Mapnik being able to do something; the question is whether you can get your data source to compute these locations. (Mapnik's responsibility would then consist of placing an icon there, no more.)

If the data comes from shape files then you're out of luck since there's no geo processing available.

If the data comes from a PostGIS database then you could mark all intersections between two bits of road like this:

SELECT st_intersection(a, b) AS geom
FROM planet_osm_line a, planet_osm_line b
WHERE a.highway IS NOT NULL
AND b.highway IS NOT NULL
AND st_intersects(a,b);

or substitute your own table and attributes instead of planet_osm_line and highway. Note that while this will be correct in one direction (a road that ends near, instead of on, another road will never show an icon) it is not necessarily correct in the other (an icon shown on the map does not necessarily mean that you can turn into the other road there). That's because the "simple feature" geometries don't have topology and cannot differentiate between two streets actually being connected (i.e. same node id in OSM) and two streets just incidentally being in the same location without being connected (e.g. due to a different z order).

You can correct for some of that by adding stuff like and ((a.bridge is null and b.bridge is null) or (a.bridge=b.bridge)) etc but it is always going to be guesswork.

In the OSM world you could get perfect results by evaluating the planet_osm_ways table which is generated when you run osm2pgsql with --slim, however this is a bit complicated.