Integromat will charge you according to the total number of operations performed by modules in your scenario. More modules and more bundles being processed in a scenario mean a higher number of operations. The amount charged for your scenarios depends on how complex these scenarios are. Simple scenarios can be run for free. More sophisticated scenarios will usually require you to upgrade to a paid subcription.
Whenever a module in a scenario is started, it is counted as one operation. The first module in the scenario is run only one time and is always counted as one operation (even if it does not return any bundle). The number of times the rest of modules are run depends on the number of bundles they must process (one run of a module per one bundle). An exception is the aggregator module which is counted as one operation per set of bundles being processed.
The following scenario connects Instagram, Image and Dropbox package modules. It shows how to download images from a selected Instagram account, resize them and upload them to a selected Dropbox folder.
Once the execution of a scenario is completed, Integromat displays in the little white icon next to the module icon the number of operations performed by each module.
In the above example, the trigger returned 3 photos and performed one operation. Each of the three photos then passed through the Resize module. Three more operations were needed to resize them. Finally, each photo was uploaded to the Dropbox folder. In order to upload three different photos, the module was run three times i.e. three more operations. The execution of this scenario required 7 operations in total (1 + 3 + 3).