It really depends on your server's capabilities in handling connection/application request plus cpu/ram power. I would say no exact figure for that.
Maybe you can set a threshold of max conn to 1000 max connection, but set the half-open session @ embryonic level/limit to 200 or less.
This (emb_limit) at least allows you to control syn request to the server (and tcp sync attack), and see if you need to increase, maintain or lower the number.
But if you expect huge traffic or many users to access it, i.e e-commerce server/application, you can probably set the emb_limit higher and set the max conn to bigger no. But start at relatively smaller than 5,000 max connection or less.
Need to consider your internet line/bandwidth, i.e huge data to download vs smaller bandwidth or the other way round, as well as your PIX capacity in handling incoming connection (i.e PIX 506E vs PIX535 in handling expected 100,000 concurrent connection).
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...