What's the maximum length of a SIM card PIN that iOS supports?
The SIM PIN has a minimum of 4 digits and a maximum of 8 digits.

This goes for the iPhone, all iOS devices - and indeed any other standards compliant GSM cellphone.

For the latter statement, please consult this standards document:

https://www.3gpp.org/ftp/TSG_T/TSG_T/TSGT_05/Docs/PDFs/TP-99185.pdf

From page 66 and on, the PIN functionality is documented. Note that the PIN data length is 8.