Ten to one is a reasonable starting point if costs are not a factor to the PBX owner, otherwise, that is a multi-faceted question......
What are the anticipated calling patterns of the users? IOW, what type of business is it?
What are the ratios of incoming to outbound calls?
For outbound, local vs toll traffic?
What types of trunking services are available?
What are trunking costs for the area where the PBX is located.....if services are expensive, the customer may have to accept a higher probability of blocking if the budget is tight......however that needs to be weighed against the potential cost to the user of those calls that are blocked.
How easy is it to have the service provider add more if you have too few?
Lots to consider.....good luck,
Dave Strang