As someone who regularly installs new electric sockets I'll give you the experts view:
1) It's difficult to have too many sockets in an apartment and too few is a constant nightmare.
2) It's not common for people renting to have new sockets installed as the cost is quite high and a lot of people don't want to spend their money on a rental palace (despite the fact they spend a lot of their money on a rental place)
3) The cost is proportional to the length of the chase (the distance from the socket that the new socket (s) are situtated, but the cost per meter falls as the number of chases (and meters grows).
4) Most of the cost is the length of the chase, a good sparky shouldn't charge you double for a double socket over a single.
5) The reason why you need multiple sockets in a flat is not just convenience, it's primarily safety. If you're running all your appliances in your living room of a single socket with a power bar, you're almost certainly going to overload that power bar, which is bad idea. Overloaded extensions are a common cause of electrical fires and fires in tall buildings are really not good news.
6) If you do decide to go ahead and get sockets installed, your landlord would be insane if they were to require you to remove them when you left (though it is prudent to get this in writing, just in case they forget to take their meds for a few weeks). Insane landlords are not uncommon in HK - But THAT insane - nah. Even the utter twats realise that more sockets in a place lacking them is a very good upgrade and not something subject to the 'taste' of the next tenant.
7) Cutting chases is very dusty, noisy and time consuming, but the dusty/noisy bit can likely be completed in a single day. After that, concreting/plastering/painting is not too intrusive.
8) If you go the extension route, get high quality extensions with cables long enough to reach your locations (don't plug an extension into an extension to add cable length) and carefully spread the load of your appliances across several sockets (which means cables running from more than one socket)|
9) What bars and restaurants often do is go the external conduit route. This means routing the conduit outside the wall rather than inside the wall.:
Pros:
Cheaper
Quicker
Less noise and mess
Reasonably easy to install means easy to remove - See cons.
Cons:
You have plastic conduit running around your walls which doesn't look great indoor
Your landlord would likely require it to be removed when you left
Ultimately, if you'd like a quote to have sockets installed (routed either inside or outside the wall), then send me a PM.
Cheers.
Conduit: Don't have any restaurant shots to hand, but here's conduit in external applications: