When there is high demand for Cosmos DB, you will need to properly scale out. In Cosmos DB settings, there is a scale setting. Cosmos DB has a unit called RU/s (request units per second), which is used to calculate cost and scalability. You can also think of RU/s as a number of transactions or requests per seconds. If many users are using the application, Dialogflow will send more concurrent requests to get the fulfillment. For example, if you have 1000 daily users, it is very likely that there will be 1 to 20 concurrent users, which means 1 to 20 RU/s. You can further estimate your Cosmos DB RU/s by going to Microsoft Request Units and the data storage calculator (https://www.documentdb.com/capacityplanner#).
The following screenshot shows the Cosmos DB scale setting:
