do you use a wireless adapter that is dedicated to your Wireshark trace? that is promiscuous mode and no data?
if not, your capture may be be influenced by your own data traffic on the adapter.
first try to install a second wireless adapter, using one for data only and the other for sniffer only.
there is no limit that "should be...."
in a heavy loaded environment with many clients and/or access points collisions can occur wich result in retransmissions.
although retransmissions must be as low as possible, in some cases this should be regarded as "normal" (for the local setup)
especially the 2.4GHz band known to be overloaded