It really depends on how long the party will be, and what time guests are invited for.
The rule of thumb is 4 to 6 canapés per person per hour. If your guests are invited for 7pm, consider that they probably will not have had dinner yet, so might eat a bit more. On the other hand, if the party is called for 9pm, most guests will have already eaten, so fewer canapés will be needed.
I prefer to be cautious and have extra canapés, since they will probably get eaten anyways, rather than run out.