You can install five halogens on one circuit if they're 240V and 150W. That's a lot of light. 13A is sufficient for this.
I'm not completely sure what you mean by a 13A fusebox or having a 240V socket. It's possible that you have an old home that has a subpanel with a 13A fuse in it and that somewhere is an outdoor 240V socket. Each of those things alone are extremely rare on newer (1960 or later) houses. Maybe you're in Europe and this is common. Whatever you do, make sure you follow your local electrical code.
What's the physical configuration of this? Are they all attached to a house? Is the whole system fixed or does any part of it move?
Stephan Samuel, April 2007