This is an interesting question, and it depends on a lot of different kinds of answers. The business architecture team here in Microsoft IT has developed a set of services that they will offer to various customers, both within IT and in the business. There is also a mapping showing which people, within the BA team, are qualified to perform each of those different services.
Therefore, to answer the question, you have to understand the demand for those services! How many people want to “buy” service 1? How long does service 1 take to perform? How many resources does it take? A little bit of math and you can get to a starting estimate of the number of folks you need. Of course, this is just an estimate. Some folks will seem interested but will never buy the service. Others will buy the service and not understand what it is, creating a situation where success is nearly impossible.
The worst case scenario is when a manager simply “anoints” someone as a business architect. That is bad for the manager, bad for the newly minted but totally unsupported architect, and terrible for the “brand” of “business architect.” By definition, everything that they do will be novice at best, and probably just wrong. Even when quality output appears, it takes twice as long as it should.
And every business stakeholder who meets them will think “this incompetent person is a business architect?” Just try to get them to buy BA services again. I dare you.
A careful Business Architecture growth strategy can be difficult to drive. If you get folks interested, you may generate a LOT more demand than you have capacity. If you generate too little demand, your window of opportunity may simply close. It’s a tough road.
How many business architects you need depends on so many things, but mostly it depends on your brand. How well have you managed the brand of “business architecture” by making sure that Business Architecture delivers a high quality service every single time? Perhaps that is the most important question for the growth of business architecture all up.