I would sugest asking sid he knows a lot about water plants

I would have thought you could have more than one medium sized one. You need to lower them as they grow as they need light to grow, if you put new ones in to deep at first they may never appear if there is not enough light? I found this out when I put mine to deep to start off, but my pond is 3ft 6" deep.

when you go to buy the lily there should be info as to what depth they are suitable for. Pygmay ones will not be suitable. You may also find some of the 'larger' ones will be too vigourous. Vague I know but there are hundreds of lilies availble for ornamental ponds now.