...or how to make scarce resources go further.
Good enterprise architects are rare. Building a high performing team of twenty is difficult, building a focused and effective team of fifty is near impossible. But you could probably build teams of 5 or 12. Larger? ... it's not architecture.
The key is the vast free resource that is available and willing to be used. First you have to make a mental conversion. Your team will set objectives for architecture and assure that they are met by your free architecture resource but they will not create architecture and you will not control the resource. Are you prepared to give up control? Are you prepared to trust someone else? Are your architects prepared to watch and criticize? Are they prepared to substitute depth of knowledge for breadth?
If you are about to embark on a major implementation then you will be spending large amounts of money with suppliers on hardware, licenses and consultancy. The suppliers have funds to invest in winning this business – for low margin business such as consultancy, it is normally a few percent of the deal value, for higher margin business such as software the percentages will be higher. The suppliers have teams of people just for the task of helping you buy their products. Why then would you do their job for them?
There is one external resource that you will need to pay for, but you probably are already. You will need access to a major analyst company to identify the right suppliers, to tell you where the markets are going, to help you evaluate supplier responses, to identify good reference sites, make calls on risk, etc.
Your architecture team is your staff, your existing major suppliers, your analyst, and the vendors bidding for the next piece of work. If you can manage this lot, why would need a larger team?