I've made a part of an watch chain containing five different models, how can I easily control the array modifier on all of them as if they were a single object?
By using drivers you can use one value to control multiple settings.
In the watch (or the first strap piece), add a custom property called
StrapLength then use that property to drive the array modifier value of the other pieces. Right click on the array count value and select
Add Driver, then in the driver properties you set it to use a single property and select the object and set the path to
["StrapLength"]. Repeat for each piece in the strap.
Then when you adjust the property, each array using the value in the driver will change at the same time.