I am successfully able to manage 4 clients connected to uMQTT broker on esp8266 AP mode(for my case esp8266 is necessary to be in AP mode so that other esp8266 should be able to connect to broker locally). Trying connecting 5th client is failed. Reading thread I understood it's because of the esp8266 limitations of memory. According to thread if I manage espconn_tcp_set_max_con() to increase connections still it will be unreliable.
As I wanted cheap solution I don't want to go for RPI hence ESP32 Broker could be the other option but I don't know how to use it with arduino like I used for esp8266. Will it be reliable to connect more clients?