Answer
There's no clearly documented hard limit on the number of addresses specifically, but since the recipient list is built and passed through underlying PL/SQL string variables, the practical ceiling is bounded by however large a PL/SQL VARCHAR2 variable can be in that context (on the order of tens of thousands of characters) — meaning realistically dozens of addresses would work fine but it isn't designed for very large recipient lists. For sending to several or many people regularly, it's generally recommended to instead create and manage a genuine distribution list at the mail-server level (such as in Exchange) and reference that single distribution-list address in the Custom Event, rather than hardcoding and maintaining a long comma-separated list of individual addresses directly inside the event itself — it's considerably easier to maintain membership changes that way, and mail-server distribution-list management can also be delegated to someone else without touching the IFS configuration at all.