I have a baseball signed by 12 members of S.F.Giants including Willie Mays-Bobby Bonds?

A ball signed by Willie Mays alone will run around $250-$300 depending on the quality. A ball signed by Bobby Bonds alone will run around $200, again varying by quality.

A ball signed by both, and then more will be worth much more. Especially if it has any established providence (You can authenticate the signatures and any good, real stories behind it).

As for being a WS ball, it is not one. The four players you named only played together for 2.5 seasons, 1968 (Bond's rookie season) - July 1970 (when McCormick was traded). Depending on other players on the ball, you may be able to narrow it down to the exact season. They did not make the pennant in any of those seasons. If you want to cross-check some other names, go to http://www.baseball-reference.com/ and go to the SFG page and look at those years for the rosters.

However, the ball still has some classic HOF worthy signatures, and if authentic, could be worth some. Value for an item like that is only what someone is willing to pay for it at the moment you want to sell it, and a reliable collectables shop should be able to get you a relatively good value, and help you get it sold if you want that.

Wow, Don't sell it.