there’s some rpc tests which tx 1 zootoshi to 2-3k t-addresses, zcash/wallet.py at 3920292b4368a42cf56c7dc16fa324b60e24a8c8 · zcash/zcash · GitHub, but there’s no indication whether this is the actual limit or not.
in the wild, we have a case for shielded to publics tx’s where 1500 was the ceiling. see Update fee calculation and thresholds so that large z_sendmany transactions are mined · Issue #1851 · zcash/zcash · GitHub, Zcash pool - zec.nanopool.org - #103 by nanopool