[SalesForce] Roll up from contract to Account using Process Builder

Process Builder and Flows are bulkified (e.g. you can safely update a field on 200 records at once), but they can't handle the situation where a single record is updated a large number of times in the same transaction (more than 12). Unfortunately, Flows and Process Builder aren't great at updating parent records in a bulk pattern, and there's no way to make something that resembles a "map" in the Flow engine. You will need to use Apex, either by way of an InvocableMethod or just writing a trigger to enable mass updates in a safe manner.