You certainly can send 20 requests in a day, although one dedicated designer working on your requests will only be able to complete one request at a time and subsequently your requests establish a queue. First in, first served.
Depending on the complexity of your request, we like to give a general guideline of at least one day per design request. This time frame will extend again if there are edit requests. Gauging a timeline for a product from a creative service definitely requires some common sense and understanding from both client and designer