Are you talking about classes generated by a Pega wizard for connectors? That is where I've seen the limit of 56 observed due to prefixes being added. I thought 64 was the general classname length limit.
This is old but interesting and relevant. It suggests unchecking classname inheritance.
No, I mean class created manually. Cant create subclass with name "M" for a class with name Sb-FW-SMEMonitoring-Work-LACard-ReceiptOfAgreementFunds as overall length of new class Sb-FW-SMEMonitoring-Work-LACard-ReceiptOfAgreementFunds-M is more than 56 symbols.