The core can be directly trained multiple times a week, since it is predominantly made up of smaller muscles with a fair amount of slow-twitch muscle fibers. A good rule of thumb is to train core on days that potential fatigue won’t limit main lifting performance.
In terms of recovery to benefit your other lifts, a good frequency for many to experiment with would be:
Beginners: Two times per a week
Intermediate: Three to five time a week
Be sure to reduce the number of sets you perform if you’re logging more sessions per week.