What would be the maximum number of signatures where it would cost about near block gas limit to verify and execute a transaction. What would be a reasonable number to limit and add other logic to a contract instead of high cost?
In all the multisig contracts I've seen, it's not that a bunch of signatures are validated at once. Rather, each participant makes a separate transaction to confirm a proposal, and once the proposal is confirmed by enough participants, it's executed. So there's one transaction per participant, and the block gas limit doesn't apply (since the transactions don't need to all be in the same block).