You know, I have never, ever built so many VPN's in a 520 that I would ever have to worry. The most concurrent peers I have had is 17. Curiously enough, the show version shows: ISAKMP peers: Unlimited
It works very, very well. The PIX is well suited to handle the tunnels.
As we all know, VPN's (depending upon SA lifetimes) will come and go. As far as Cisco's recommendations???