There are 88.
Seaborgium is an artificial element, not found in the nature.
An element is an element, whether it is found in nature or created in a lab.
Uranium, element number 92, is the largest naturally occurring element. However, francium, 87, and astatine, 85, along with any element with an atomic number larger than 92 are not found in nature. They are produced in the laboratory. If they do occur naturally they are in exceedingly small quantities. So to answer your question "How many elements can't be found in nature?" about 16 or a few more.
Americium is an artificial element, don't exist in the nature.
Roentgenium is an artificial element, not found in the nature.
helium
Sodium
The element with 161 neutrons in period 7 that is NOT found in nature is astatine-215. Astatine-215 is a synthetic element that is artificially produced in laboratories and is not stable enough to exist in nature.
Manganese is an element. It is not found as a free element in nature; it is often found in combination with iron, and in many minerals. Manganese is a metal with important industrial metal alloy uses, particularly in stainless steels.
Hydrogen has the lowest atomic number, and is not found in nature as individual atoms, but is found in nature as the diatomic molecule, H2.
Sodium, being a reactive element In group 1, cannot be found as element in nature. One of its common compounds is rock salt.
Hydrogen is found in nature primarily as a compound, such as in water (H2O) or hydrocarbons. Pure elemental hydrogen is rare in nature but can be found in some volcanic gases or certain chemical reactions.