How many instant hot water heaters do I need?

Most units that size will have a single combination boiler to do hot water and heating. 

Dedicated units are useful if the layout is far flung in which case you might have one for bathroom and another for the kitchen.

Normally, just one for the whole house where you would usually put a tank. If the pipes are long you might add units at the kitchen and bathroom as needed so you're not wasting water waiting for the hot water to arrive.