ChrisF is totally correct; it's not the number of bulbs you should worry about, but how much load they place on the circuit.
Chris covered UK numbers, so I'll stick with U.S. Most lighting/appliance circuits in newer homes are 120V 15A, with certain branches required to be 20A and others able to be beefed up to 20A as needed by the design or by demand. As Watts = Volts * Amps, a 15A circuit will max out at 1800W, and a 20A circuit will max out at 2400W. That's before the breaker trips; when designing the circuits and choosing your lighting, you should not exceed 80% of that, to avoid nuisance trips due to transient overloads. So, a 15A circuit should not have more than 1440W load placed on it at once, and a 20A circuit should not have more than about 1920W.
So, to calculate how much "spare" capacity your circuit has, take the amperage rating of the circuit, multiply by the service voltage of "single-phase" power in your jurisdiction (120V for most of North America, 220-240V for Europe, Africa and most of Asia, 100V in Japan), then multiply by 80% (.8). Now, subtract 120W for each light fixture that is on the circuit except the one you are replacing (this is from a rule of thumb stating no more than 12 lights on one 15A circuit), and 160W for each outlet receptacle (from another rule of thumb stating no more than 9 outlets on one 15A circuit). Current U.S. code discourages use of a circuit for both lighting and outlets, but as of the late 80s it was perfectly acceptable in the U.S. and is still doable just about everywhere (in a house as old as yours with so many people's hands in the wiring, who knows what a particular circuit may power). The remainder is the spare capacity this circuit has, and the total wattage of the bulbs in your new light fixture should not exceed this.
The wattage in your light fixture is calculated similarly; multiply volts (120 for the U.S.) by the maximum amperage rating of each socket in the fixture. This would allow you (or a future homeowner), not knowing the actual wattage of a set of bulbs you'd put in earlier, to simply buy what the light fixture sockets say they'll support. While you can downgrade the bulbs you use (13W candle-style instead of 25W or 40W) and put the light on a dimmer switch to reduce the actual wattage draw of the fixture, it's not a really good idea to put in a fixture that would blow the circuit if you loaded it to its full rating.
If the circuit would be overloaded by your new light fixture (or is already, with or without it), you can do one of two things; upgrade the existing circuit, or split off some of that circuit onto a new one. Both will probably require an electrician to come out to make the connection to the panel (DO NOT TOUCH the wires behind the panel breaker switches or fuses yourself; from lowest-severity up, you can be fined by your local jurisdiction, invalidate your home insurance policy, start a fire, or kill yourself).
Now, if your home is over 100 years old, odds are it still has a fuse box, and it may even have knob & tube wiring. There are special considerations to take into account here; fuses can be as small as 5A, and K&T is rated for a max of 10A. Again, NEVER over-fuse a circuit, even if the fuse fits; that's how electrical fires happen especially with K&T. Changes to the number of circuits, or to the rating of a particular circuit, can be costly, as it is illegal to modify wiring that isn't up to code without bringing it up to code. This means replacing K&T with Romex, replacing the fusebox with a breaker panel, installing peroper switches and receptacles (as of 2011 all outlets must be tamper-resistant even if you don't have kids, outlets in "wet" indoor areas like bathrooms and kitchens must be GFCI, and outdoor outlets must be weather-resistant), etc etc. This is the worst-case scenario, but in a house from 1912 it's certainly possible that adding capacity for this new light fixture may require a whole-house rewire to bring it up to current NEC2011 code.