Ok, the input (230v, 50hz) doesn't matter so much, or rather, it's a given that adapters you buy in Singapore will have those specs.
What you wanna look at is the DC output. Both numbers (voltage and current) are important.
Voltage: 9.6v or approx 9v is what majority of pedals use (Boss and most other brands). Other less common voltages for pedals are 18v, 15v, 12v. This is NOT interchangeable unless stated on the pedal that it accepts a range (9-18v for example).
Current: 200mA (200 milliamps) = 0.2A (2 amps). 200mA is the standard for those adapters bundled with Boss pedals. It can power a few analog pedals, but it's really not much. Some of the "better" adapters like OneSpot or Godlyke have output in the range of 1700 to 2000mA. There's nothing wrong with using a higher rated adapter, but providing too little current to a pedal will result in weird behaviour or it simply not working.
How many pedals you can power up depends on the current draw of the individual pedals. Some analog pedals take as little as 3 to 4mA. Some digital pedals require 300+ mA, up to more than 1A. This website has a list of the power required by many common pedals:
http://stinkfoot.se/power-list.
