From the POE Wiki as of Sentinel League 3.18:
https://www.poewiki.net/wiki/Raise_ZombiePlayers can have a maximum of 18 (19 if using talisman, 20 or more with legacy items and/or specific str-stacking build ) zombies at once:
- 6 from the level 20 skill gem (up to 8 zombies from level 34 gems, but not practical to reach such gem level)
- 1 from Jun's veiled mod on helmet
- 1 from amulet (2 from Eyes of the Greatwolf that has the correct implicit +1 to maximum number of Raised Zombies, which would be doubled due to Eyes of the Greatwolf's explicit mod; or specially crafted talisman listed above)
- 2 from dual wielding Midnight Bargain
- 5 from The Baron with 2500 Strength (1 zombie for 500 Strength unless using legacy item; 3000 Strength may be reached by some build)