16 Relay Drivers with 6501

I used to drive 4 5v relays with a resistor (10K) and a transistor (bc548), using a diode in the relay coil.

But now I was trying to drive 16 relays using 16 D.O. of my 6501. The problem is when i have like 9 or 10 relays in high state I can't turn another relay on.

All lines are at default (open drain).

I'm not sure if I reached some limit of the device (overall max current in open-drain?)