AFAIR, the rule of thumb with the older style Marklin incandescent bulbs is that you allow 1va of current per bulb.

Therefore, if you have a 40va lighting transformer, that would power 40 bulbs. The voltage will not vary, most Marklin transformers deliver between 16 to 18 vac. It is the current draw you need to worry about.

Something that you may be interested in, is that there is a company in the US that sells a led based 'bulb' (for want of a better word) as a direct replacement for the Marklin bulbs. LED's draw much less current than a normal bulb, you could probably get around 4 to 5 LED's per 1va of current. And they produce less heat than a bulb.

http://www.trainaidsa.com/

Look under Marklin, for the C5 bulbs, which are a direct replacement for the 60000 Marklin bulbs. And you can get them in 6 different colours. They also include the required current limiting resistor.