I have 12 in the "tiger tank" in my sig. and they are thriving. They are kept strictly as a species tank (I personally have had better luck keeping tigers separately from other fish...Less stressful).
Is the twenty gallon tank you are planning on using a "long"? If so, you may be able to keep a few more than if it is a "standard". The 20 long has the same surface area as a 29 gallon but is not as "tall".