How to assemble PCBs when SMT machine doesn't have enough feeders?
Most contract fabrication houses would load up all their feeders and then make the run of boards for those components and then cycle back through the boards for the next load of components in the feeder. There are practical limits however including how many boards the factory can stage at one time, how long the solder paste stays able to accept additional components and how long the feeder change over takes.

In some factories you may see several pick and place machines in series on the line so that multiple sets of feeders can be active at a time to accommodate boards with very high BOM line item count.