Fair enough. All four of them.

How many do they need ?

They have the two largest smartphone manufacturers outside of Apple and RIM, in HTC and Samsung.

Among the large manufacturers, all they are missing is Sony Ericsson and Motorola. And I believe only Motorola has hinted that they are Android only.

And Nokia is still larger (not for long) than #2 and #3 in terms of worldwide sales.

Actually, they could line up a couple dozen WP7 phone manufacturers and still fail. What they need is not manufacturers, but people buying and using the phones. This is not happening.

They also need developers creating content for the platform, which isn't happening either. Chicken and an egg there, though.

>They also need developers creating content for the platform, which isn't happening either. Chicken and an egg there, though

It is, WP7 marketplace already has around 15,000 apps and is growing faster than the iPhone and Android stores at a similar stage.