Subscription number for MMOs are typically hard information for joe-smoes to get.
If I had to guess, I'd expect initial sales for Rift to be quite high. Not so much because I'm hyped into believing it's such an amazing game, but because it's had an above average budget for an MMORPG with plenty of marketing dollars spent to advertise for it. It's also had a very stable beta, which should provide positive word of mouth.
If I had to guess, and there's a fair amount of bull **** in this claim, a conservative low would be 500k initially, and 700k being more reasonable. 1 million is stretching it, but possible with the amount of money being spent to promote Rift. This is based on the above in combination with data from other similar MMORPG release (AoC and Warhammer both hit 700-800k initially). LotrO did far less peaking with 300k, but I don't feel that is as predictive of Rift.
There's also a high chance of decline in subscriptions after launch, which is typical for many recent MMORPG releases. Some decline isn't exactly disastrous, but I'd say the game has failed to meet investors expectations if it hemorrhages about 50% of the playerbase in a few months.