The vast majority of civilian aircraft and powerplants flying worldwide are manufactured in the USA or Canada using almost exclusively Standard hardware, usually conforming to aircraft Mil Spec (MS) or AN specifications. From that perspective, the answer to your question is that any given aircraft will most likely use all Standard hardware.
This is even true for some European built aircraft such as the Reims F172 manufactured in France, or the Italian SIAI Marchetti F260. These aircraft use standard hardware, such as AN bolts and washers.
As Peter Kämpf described, there certainly are aircraft with a combination of hardware types. There are also a significant minority of aircraft that use exclusively metric hardware.
In any event, maintenance personnel will often have documentation detailing the exact type of hardware used on the airframe and engines. Older aircraft may not have this documentation, for example for a Beech Staggerwing, but the vast majority of such aircraft will use Standard hardware.
Most aircraft mechanics do not have any metric tools in their aviation toolboxes. There will be an exception, obviously, for those that do work on the minority of aircraft with metric hardware.
Speaking from my experience of having worked at remote airports, on vintage aircraft, most toolboxes reflect the use of Standard hardware. The metric box is typically reserved for use on automotive projects. That does not mean there are no exceptions; there are.