Not all of the 8 billion humans on Earth are able to have at least one shower per day because there isn't enough water for everyone to do so.
Local water availability depends on a number of factors: rainfall in the region and adjacent regions and the ability to store the water for periods of low rainfall. Some people may want to raise the matter of ground water stores which can be accessed via wells or water bores. Ultimately, all water sources on land depend on rainfall.
In most affluent countries people can take at least one shower per day. In regions with prolonged severe drought, as the southern and western part of the US are experiencing were water storages are very low resulting in some people reconsidering there current water usage, with the aim of reducing such usage.
The amount of water a person usages during a shower depends on the flow rate of water through the shower head and the duration of the shower. In some jurisdictions people are encouraged to change their shower heads to low flow heads, which reduce the amount of water they discharge.
Different website give different quantities for the amount of water a shower head releases. Taking one at random, in the US,
The standard shower head produces about 2.5 gallons per minute (9.5 L/min). That is 25 gallons of water every ten minutes (95 liters every ten minutes).
If this is typical, a person uses 175 gallons (662.5 L) of water per week for showers. During the course of one year, that amounts to 9125 gallons (34 542 L). In a city of 100 000 people, that's 912.5 million gallons (3454 ML) of water per year - a significant amount of water.
If everyone took a 5 minute shower, instead of a 10 minute one, the water consumption would be halved which would greatly affect the region's water stores and make a water crisis situation, as currently exists in many parts of the US, a little more tolerable.
One person may not make a difference, but the collective action of a large number will. By reducing water consumption you are doing something for your location community, not for others in other countries.