Twitter had dinner at the SF office but they discontinued it after they realized that the amount of people that used it didn't justify the cost of operating the kitchen that late. They replaced it with a take-out option, prepared by the kitchen. I really like this since it still allows a useful perk but gives people the option of where/how the take advantage of it.
A DL stream is a single partition if you are thinking in terms of kafka. We stitch them together as a partitioned stream a la Kafka using a different system. So, to answer your question, no, there is no limit to the number of DL streams you can put together into a bigger logical stream, we have some very large ones.
Edit: it's not free but highly subsidized.