As per 3GPP 29.272 spec, there is no max limit on APN configuration however a LTE UE can have maximum 8 data radio bearers at eNodeB and each data radio bearer is mapped to corresponding EPS bearer in EPC. All eight bearers might be combination of default and dedicated bearers.
Usually, I have seen one APN for internet and other one for IMS. In this way, total three bearers (1 internet, 1 IMS signalling and 1 for VoLTE call) will be used for these two APNs. Remaining five bearers can be used in any combination of default and dedicated.