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. 


if (sig.size() < 9) return false;
    if (sig.size() > 73) return false;
    
    if (sig[0] != 0x30) return false;
    
    if (sig[1] != sig.size() - 3) return false;