len

With a string:

{{ "ab" | len }} → 2
{{ "" | len }} → 0

With a slice:

{{ slice "a" "b" | len }} → 2
{{ slice | len }} → 0

With a map:

{{ dict "a" 1 "b" 2  | len }} → 2
{{ dict | len }} → 0

With a collection:

{{ site.RegularPages | len }} → 42

You may also determine the number of pages in a collection with:

{{ site.RegularPages.Len }} → 42