It is not quite clear from your description how coil accounting is organized.
In case when each record in "Coils" describe a single unique coil, and each record in "Coil Usage" describes consumption of a single unique coil with a relation from "Coils" to corresponding record in "Coils Usage", you can get the quantity on hand by defining a filter that would select only records in "Coils" that have empty value in a field from relation to "Coil Usage" - the number of selected records is the number oon hand.
If each record in "Coils" describes a multiple number of coils purchased at the same time (say in field "Count"), and each record in "Coil Usage" may include consumption of multiple coils, you would just manually subtract number of used coils from the "Count" field in the record in "Coils", and link a record in "Coil Usage" to the record in "Coils". To get current number on hand you create a report view with a sum of values in the "Count" field of records in "Coils".
If in the last scenario the coils may have different types, to get current number on hand of coils of a given type, you would need to apply a filter that selects only records for coils of that type.