I take it you mean amount of a specific enzyme, not total enzyme....
The amount for a specific enzym can be regulated in a few ways:
A gene can be Induced by a rising concentration of the substrate.
Think for instance about drinking alcohol:
Once the ethanol ("booze alcohol") enters your system, it will be noticed and acts as an activator for the gene coding for Alcohol Dehydrogenase . The higher the concentration of alcohol, the more active the gene will be but only to a certain maximum....
Once the Dehydrogenase is made, it will break down the Ethanol (convert it to AcetAldehyde), thereby lowering the concentration, and relieving the "induction pressure" on the gene that encodes for it.
Incidentally, Alcohol Dehydrogenase is one of the fastest working enzymes known...
As Acetaldehyde builds up, it induces the gene(s) that code for Acetaldehyde Dehydrogenase , which will convert Acetaldehyde to Acetic Acid (VInegar).
Here's an excerpt from the Biochemical Pathways poster by the way..:
you can find the whole map here :http://biochemical-pathways.com/#/map/1
Not easy to find an answer off the top of my head, but an enzyme can also be its own Inhibitor: it blocks its own transcription, usually by blocking the operator, located upstream from the Gene.
I'll keep this short, but this article might tell you more:
If you do, have a look at the Lac-operon ....