In the kitchen, have a lot. If you have a double socket every 1m in a row 150mm/200mm above the worktop, it is easy and quick to install, and there will always be one handy for your kettle, mixer, toaster, coffeemaker, juicer, radio, phone, microwave, tin opener, curling tongs etc.
In the same row, have switches for all the below-worktop appliances that you have, or might one day want, and for the above-worktop outlets like your extractor, boiler, TV, under-cabinet lights, wall-mounted microwave and espresso. Appliances are often 600mm wide so try to space the outlets evenly. You will also need cooker outlets, usually 30A dedicated circuits. I have one on each side of the kitchen to allow for repositioning at whim. You might also consider a dedicated freezer circuit on an RCBO so the freezer does not go off if there is a trip, for example if the kettle falls in the sink or the puppy chews through a flex.
For your appliance switches, you can buy them with or without neons, and, if you wish, engraved with the appliance name, ssuch as freezer/dryer/cooker hood/boiler.
You can have anonymous switches hidden inside cabinets behind piles of cornflake packets and ironing boards if you want to, but the day your tumble-drier or chip-fryer overheats and starts to smoulder, you will wish they were readily visible and their purpose obvious. Remember the person dealing with the flaming appliance might not know where you have hidden the switch.
In other rooms, start with a double in each corner of the room, and along walls at least every two metres. The one near a door should be behind the door so there will be no need to trip over a flex. Halls and landings, at least one at each end and one half-way along, but not so near the stairs that you will fall down them when you trip.
One near the loft hatch so you can take a drill and a lamp up there, unless you are providing loft sockets and lighting.
Above desks and workbenches, numerous outlets. You can have them fitted in a sort of dado rail that you will have seen in hospitals, that has isolated chambers for phone or LAN cables, which must be separated from mains voltage cables.
You will know that modern standards are to have sockets and switches at least 450mm above the floor. if you grew up in an old house you will be disappointed that you are no longer able to crack sockets by banging them with the hoover, but anyone under 80 will become accustomed to these heights and will think that low ones look strange. If you are planning ever to become old, fat, pregnant, disabled or to have a bad back or knees, you will find low sockets a burden.