Here, you're combining two rather distinct models, which is where WiFi router intuition fails.

In contrast to a LoRaWAN gateway, a MySensors gateway with rfm95 is essentially stateless. It doesn't keep track of downlink windows, keys, queues, per-node sessions, or anything like. Messages are simply forwarded up and down. Therefore, unlike a WiFi AP, an ESP8266/ESP32 does not have a growing "connection table." Because of this, the 253 node limit is mostly a protocol decision rather than a RAM/storage limitation.

The actual hardware limitations are shown elsewhere:

airtime. The modulation of LoRa is sluggish. Collisions and duty cycle constraints will catch up to you far before RAM does when there are a lot of nodes.

buffers and sketch size. rfm95 + Small packet buffers are used by MySensors; the number of nodes does not linearly increase memory use.

bursts in CPU burden. If several nodes communicate at once, the ESP8266 may choke, however this is a temporal issue rather than a storage one.

ADR, security, scalability, and multi-gateway roaming are among the issues that LoRaWAN resolves. With long-range radios, MySensors over rfm95 is more akin to a basic star network. less expensive, easier, but with less assurances.

Therefore, an ESP-12F gateway can "handle" hundreds of nodes with ease from a hardware standpoint—as long as they communicate seldom. It will shut down before flash or RAM ever do if you attempt to make it act like a dense LoRaWAN deployment due to airtime and physics.