What dimensions? If standard, I would say no more than 5. They can be messy.
That said, delhezi are a great option. If you plan on getting a larger tank then just get whatever suits you. They are all beautiful! A 75 will be suitable for small specimens of any kind, and it will be a while before most species (if purchased young) will outgrow it. Plan on an upgrade in my opinion--big tanks are more interesting to watch, allow for more interesting scapes, and, of course--hold the biggest and most awe inspiring polypterids.