Why does upwards lightning use only one path but downward lightning uses multiple paths?
Upwards stroke happens when the stepped leader and the traveling spark have already connected. This path then becomes much more conductive than any other paths where the connection hasn't happened yet—because the latter ones have gaps of non-ionized air inside them. This highly conductive path shunts all the remaining stepped leaders, lets the charges quickly cancel out, and thus basically stops propagation of the leaders.