"how many WS-X6704-10GE can 6500 support"
7.
"or is there somekind of limit on the 6500 especially the module being 10G?"
No limit beyond what slots can be used for those cards and limits set by power supplies being used (and their usage mode). The power question, is likely the major question that needs to be answered.
Don't know if Cisco still offers it, but they used to have a tool on their web site where you could "build" a 6500, with various cards and power supplies and determine if power requirements would be met.
In your case, you show almost 1300 Watts being available. There should be another power command to indicate current card draw. Seeing what your existing 6504 card is using now would give you a good idea whether you have sufficient power for another 6504.
BTW, in theory, with two 6504 cards, you'll likely exceed the sup's PPS capacity. In practice, this may not be a problem, much depends on just how "busy" all your ports will be concurrently.
If you do overrun the PPS capacity of the sup, I recall (?) your 67xx cards might all accept DFCs. (BTW, DFCs can be pricy.)
"Fortunately" all your installed cards are fabric capable, but I recall (?) one or both of your 65xx cards are classis bus and CEF256 (NB: your 6700 cards are CEF720). I further recall (?) that with any classic bus capable cards in the chassis, the 6500 will use that bus first (if available) for the card. I recall (?) there's a configuration command to prefer the CEF256 connection. If so, with your installed cards, you would want to prefer using the fabric connection. (I also recall [?], when using fabric only connections, effectively the sup's PPS rate doubles.)
