Three APs, two running ROS6, one running ROS7:
$ snmpwalk -v2c -c public miaap .1.3.6.1.4.1.14988.1.1.1.3.1.6
MIKROTIK-MIB::mtxrWlApClientCount = No Such Object available on this agent at this OID
$ snmpwalk -v2c -c public kitchen .1.3.6.1.4.1.14988.1.1.1.3.1.6
MIKROTIK-MIB::mtxrWlApClientCount.6 = Counter32: 5
MIKROTIK-MIB::mtxrWlApClientCount.7 = Counter32: 4
$ snmpwalk -v2c -c public officeap .1.3.6.1.4.1.14988.1.1.1.3.1.6
MIKROTIK-MIB::mtxrWlApClientCount.6 = Counter32: 3
It looks like mtxrWlApClientCount is not supported in ROS7, was it replaced by something or is this somehow a config issue?