I'd say it depends how quickly you want it to cover the tank, and of course how much you want to spend!
When you receive them, you'll want to remove the pot and the fibre medium, then gently separate each plant into at least 4 or 5 sprigs and plant them separately. How far apart you plant them is up to you: further apart will be cheaper because each plant will cover more floor space, but it'll take longer to grow and fill the gaps. I reckon it'll take less time than you'd think, so my instinct is to buy fewer plants.
You need to estimate how much area you think each pot will cover once you've split it into sprigs, then divide your total floor space by that. For example, if you think each pot will cover 10cm x 10cm (i.e. 100cm²), then your total floor space of 60cm x 45cm (i.e. 2,700cm²) would need about 27 pots. But if you reckon you can stretch each pot to cover 15cm x 15cm, you'd need fewer than half that many. In the latter case it would look a bit sparse for a few months, but it would get there eventually.