Currently, the multisig descriptor constructions are limited to 16 keys only possibly because of template parsing reasons. Under p2sh context, such scripts would be invalid anyway because of 520-byte rule, but we can support up to 20 keys for CHECKMULTISIG in witness script context. 


{
    static const CScriptWitness emptyWitness;
    if (witness == nullptr) {
        witness = &emptyWitness;
    }
    bool hadWitness = false;
    set_error(serror, SCRIPT_ERR_UNKNOWN_ERROR);
    if ((flags & SCRIPT_VERIFY_SIGPUSHONLY) != 0 && !scriptSig.IsPushOnly()) {
        return set_error(serror, SCRIPT_ERR_SIG_PUSHONLY);
    }
    
    
    std::vector<std::vector<unsigned char> > stack, stackCopy;
    if (!EvalScript(stack, scriptSig, flags, checker, SigVersion::BASE, serror))
        
        return false;
    if (flags & SCRIPT_VERIFY_P2SH)
        stackCopy = stack;
    if (!EvalScript(stack, scriptPubKey, flags, checker, SigVersion::BASE, serror))
        
        return false;
    if (stack.empty())
        return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
    if (CastToBool(stack.back()) == false)
        return set_error(serror, SCRIPT_ERR_EVAL_FALSE);
    
    int witnessversion;
    std::vector<unsigned char> witnessprogram;
    if (flags & SCRIPT_VERIFY_WITNESS) {
        if (scriptPubKey.IsWitnessProgram(witnessversion, witnessprogram)) {