The default shard count is one now, and it was five

7.0 changed the default primary shard count from five to one, which is a considerable behavioural change hidden in a release note.

# 6.x: an index nobody configured had 5 shards
# 7.x: it has 1

GET /_cat/indices?v&h=index,pri,rep,docs.count,store.size

# and the sizing rule of thumb that has not changed:
#   aim for 10-50 GB per shard
#   shards are not free: each is a Lucene index with its own overhead

The old default was widely agreed to be wrong — five shards on a two-gigabyte index is five times the overhead for no benefit, and over-sharding is the most common cause of a slow small cluster. The change only affects newly created indices, so a cluster upgraded in place keeps its old layout until the next reindex. Shard count cannot be changed after creation, which is why this is worth deciding rather than defaulting.