This is a question from ggtree user. In ape and phytools, it’s easy to label edge using the edgelabels function.

set.seed(1)
tr = rtree(30)
library(ape)
plot(tr, main="ape")
edgelabels()

I don’t see any necessity to label edge numbers, as they are meaningless. The number is labeled as the row index of tr$edge, and edge can be uniquely mapped to child node. If we need to relate something to edge, we can relate them to corresponding child node. Node is a central hub in tree annotation in ggtree.

To label edge number, we can also attach the edge number to node and then label them as a node attribute.

library(ggtree)
p = ggtree(tr, ladderize=F) + geom_tiplab() + ggtitle("ggtree")
edge=data.frame(tr$edge, edge_num=1:nrow(tr$edge))
colnames(edge)=c("parent", "node", "edge_num")
p %<+% edge + geom_label(aes(x=branch, label=edge_num))