Resource Blocks are units of compute capacity in Tableau Online. Resource Blocks run Tableau Prep Conductor flows. Each Resource Block can run one flow at a time so the number of flows you can run concurrently is equal to the number of Resource Blocks that you have.
When you licence the Data Management Add-on for Tableau Online, you must purchase at least one Resource Block. To purchase more Resource Blocks, contact your account manager (or go to the Tableau pricing(Link opens in a new window) page for more information).