How does a field know when to make how many particles?

You cannot just "dump energy" , you have to model with the interactions.

Example.

The proton proton collider distributes the energy of the scatter according to quantum mechanical formulas, to the constituents of the protons (quarks and also the sea of quarks antiquark gluon) according to specific interactions , which have a probability of being expressed, since it is the quantum mechanical framework. This process has been modeled in the Monte Carlo simulation data, and the data fitted. Where there is a bump, i.e. the monte carlo has not predicted the shape, there is a discovery, as the Higgs discovery.

The straight lines under the bump are monte carlo background according to the model with various error assumptions.