The Transformer is the power supply for dollhouse wiring; it changes the household voltage (110v in the USA) to a voltage that works with dollhouse lighting (12v).  

A transformer's output is measured in "Volts" and "Amps" Volts: electrical pressure (see "Power Basics"). Dollhouse lights are sized to run at 12v so any transformer that is hooked to a dollhouse must deliver 12v of power. Look on the transformer's info tag  it will say "Output: 12v" or "12VAC". As I look through my drawer of leftover transformers, I find several transformers that are rated 3.7V (from cell phones past), many that are rated 9v (they would hardly make a dollhouse lightbulb glimmer), and one rated 32v (from a computer printer). But I did find one 12v transformer. This transformer would work to run some dollhouse lights... but how many? Amps: Amps is a measure of electrical volume (see "Power Basics"). In dollhouse wiring, amps is the rating that tells you how many lights you can power with your transformer. 12V "Grain of wheat": 60milliamps My 1300mA transformer could power (21) 12V "grain of wheat" bulbs (1300/60 = 21.66) or (38) 35 milliamp bulbs... sounds respectable! These are the bulbs used in most simple fixtures. Micro bulbs are used in chains (in electricity, it's called "series") where they are strung together in a line and their voltage draw is added together... so (4  6) 3V bulbs in series or (8  10) 1.5V bulbs in series can be used on a 12V source. Series wiring is the "one bulb goes out  they all go out!" style that was used in Christmas tree strings for years. You will find this bulb in chandeliers and dollhouse Christmas tree strings. When this kind of load is used you must add 15 milliamps for each bulb. Some chandeliers draw 150 milliamps. My 1300 milliamp transformer in a house that plans to use 3 chandeliers might only have 12 single bulbs of left over capacity  now I can see that there is a limit. 

Other considerations: Short Circuit: Your very best protection from the damage caused by a short circuit is a transformer with a circuitbreaker. This can save you many hours of tearing your dollhouse apart to find the hidden break in the circuit. I recommend dollhouses always be protected with curcuit breakers 

Sizing Transformers: Transformers are most often rated in "Watts". To translate milliamps into watts, multiply by volts/1000 or use the rule of 75 (one watt at 12Vcan supply 75 milliamps of load with a 15% safety margin built in) 1 watt = 75 milliamps = (1) 60 milliamp lightbulbs or (2) 35 milliamp bulbs If you need to go larger than that, wire the house with two or more unconnected circuits, and power each with its own transformer. 
Links  
"How does a Transformer work?"  "What is Electricity"  
"Inside a Power Cube Transformer"  "What is a circuit" 