The Count of Things

Bluebulb Projects' The Measure of Things presents:

It's about the number of Submarines in the US Navy.

(a.k.a. Subs) (includes all types, all classes) (2015 figures)
The United States Navy maintains a fleet of 72 active submarines. A typical submarine goes to sea with a crew of about 140.

It's about four-fifths the number of Keys on a Piano.

In other words, 72 is 0.820 times the count of Keys on a Piano, and the count of Keys on a Piano is 1.20 times that amount.

(Moden layout; A₀ to C₇)
Most modern keyboards have 88 keys, comprising seven octaves and an additional major third. A full-size, acoustic piano may have as many as 12,000 separate parts.

It's about one-and-three-tenths times the number of Countries in Africa.

In other words, the count of Countries in Africa is 0.770 times 72.

(2016 figures) (sovereign states recognized by the UN only)
The continent of Africa includes 54 sovereign countries recognized by the United Nations as of 2016. It is the second-most populous continent in the world, with 1.1 billion inhabitants.

It's about seven-tenths the number of Wolves in Yellowstone National Park.

In other words, 72 is 0.6920 times the count of Wolves in Yellowstone National Park, and the count of Wolves in Yellowstone National Park is 1.450 times that amount.

(a.k.a. Henihco'oo; a.k.a. Héetíhco'oo) (December 2014 figures; Wyoming Park area)

It's about one-and-a-half times the number of Teeth in a Great White Shark's Mouth.

In other words, the count of Teeth in a Great White Shark's Mouth is 0.670 times 72.

(Carcharodon carcharias) (average; including developing rows)

It's about three-fifths the number of Elements on the Periodic Table.

In other words, 72 is 0.610 times the count of Elements on the Periodic Table, and the count of Elements on the Periodic Table is 1.640 times that amount.

(a.k.a. Periodic Table of the Elements, a.k.a chemical elements) (confirmed elements as of 2016)

It's about half the number of Union Stations in the United States.

In other words, 72 is 0.5330 times the count of Union Stations in the United States, and the count of Union Stations in the United States is 1.880 times that amount.

(a.k.a. Union Terminal; a.k.a. Joint Station; a.k.a. Union Depot) (2016 figures)
Typically shared between multiple railroad companies and stations, there are about 135 "Union Stations" in the United States. Of these, the busiest station is Chicago Union Station, which handles 120,000 passengers each year.