As many as possible to be honest.
Give them plenty of little nooks and crannys at various levels if possible.
Give them various layers of rock and / or wood. Eg. Layer up pieces of thin slate to create little cave type things. Lean up some against the side of the tank, little pieces of bogwood that create natural caves.
Plenty of low level and mid level plants. So they can get under plants, hide within them etc.
Then if possible some dense planting that reaches up to the surface giving them a bit of cover when they go for air. EG. A lily, or straight vallis, allow it to cover the surface a bit, floating plants like frogbit are good too.