Welcome to the forum.
Tramadol LD50 (Source: Wikipedia)
228 mg/kg (LD50, Rat, oral)
350 mg·kg (LD50, Mouse, oral)
200 mg·kg (LD50, Mouse, s.c.)
LD50 means that with this amount 50% of experimental animals died. You can estimate how much you need to have a 50% chance that in case the human body reacts similar to the meds you want to OD on.
"Extended release" sounds like the pills contain sth so that that active substance (here Tramadol) is not released immediately and fully which makes it harder to OD on - almost impossible.