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How many Process Builder processes are still active?
Setup path: Setup > Process Automation > Process Builder (or type Builder in the Quick Find box). Sort the Status column for Active.
Salesforce still lets you edit, activate and deactivate existing processes. In the API, a Process Builder process has the process type Workflow. This query counts the active ones.
SELECT COUNT() FROM FlowDefinitionView WHERE ProcessType = 'Workflow' AND IsActive = true
Keep the WHERE filter. FlowDefinitionView does not support queryMore(), so a query with no filter can fail in a large org.
Sources: Workflow Rules & Process Builder End of Support (Salesforce Help); Process Builder (Salesforce Help); FlowDefinitionView object reference (Salesforce Developers).