Is it likely that FTX/Alameda currently have >50% voting power over Anthropic?

Extremely unlikely. While Anthropic didn't disclose the valuation, it would be highly unusual for a company to take >50% dilution in a single funding round.

What happens to those $500 mil shares depends almost entirely on Anthropic's governing documents, which are not public AFAICT.  It could be they are entirely non-voting class shares.  The FTX estate/bankruptcy trustee will try to liquidate them, i.e. will try to sell the shares.  Whether there are limitations on who they can sell the shares to also depends on Anthropic's governing documents.  I know Anthropic has some bespoke governance structure, but I don't know the specific terms.

From the little I know about Anthropic, getting anything like dollar-for-dollar on those shares is going to be very hard.

Also, my guess is that these shares were held by SBF and he just intermingled his funds with FTX's based on the leaked balance sheet.  Which isn't to say that they won't be taken by the FTX estate - very likely they will - but it's a complication because they'll have to prove they can reach SBF's personal assets first (at this point, won't be hard).

If the market value of that stake is less than FTX paid, the debtor-in-possession's first thought is going to be whether this is potentially avoidable as a fraudulent conveyance. I don't have the desire to explore that possibility, but the sum is large enough that the DIP could be interested in an argument that FTX overpaid while insolvent and was motivated by ideological reasons rather than business judgment. Otherwise, the equity stake will presumably be sold.