One delimiter, one MPDU, and padding to a four-octet boundary.
Reading a capture
The delimiter is what makes aggregation robust rather than brittle. A receiver that fails to decode one subframe can still find the start of the next, so a single corrupt frame costs one frame rather than the whole aggregate.
Subfields
MPDU Delimiter — Four octets marking the start of a subframe and giving its length.
Padding — Zero to three octets, rounding the subframe to a four-octet boundary.