In general you will need one filter for each phone device (including normal phone handsets, Sky TV Decoders, alarm panels etc) you have in the house.
When you purchase a router from us, it will normally include at least one filter.
If you need more, come into our Napier customer hub and we'll hook you up. If you're not in Hawke's Bay - get in touch and we can get some out to you. A small courier fee may apply.