There are people who might need more than one will, if they have assets in more than one country and are a citizen of both countries, according to the Financial Review in "Double trouble for dual nationals."
The problem is that some countries have strict laws about who can inherit certain property. There are laws about how much of an estate must be given to a spouse and to children.
Most countries do not allow deviation from these laws, even for people who do not live there full-time and who have a valid will in another nation.
Even if your will is valid in the U.S., it is possible that another country where you hold assets could invalidate it for the property you hold in that country.
If your estate might be subject to the laws of more than one nation, make sure that your estate plan is valid in all of the nations where you own property. If that does not seem possible, then have separate estate plans for the property in each nation.
An estate planning attorney can advise you on creating an estate plan or plans based on your unique circumstances.
Reference: Financial Review (Sep. 20, 2017) "Double trouble for dual nationals."