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But what's the maximum number of host can I have in a single broadcast domain?? taking into account QoS, network congestion etc...??
That's the 64 bit question, and "it depends".
Broadcasts have two principle adverse effects. The consume bandwidth. They force hosts to examine them, even if they are not relevant to the receiving host.
Broadcasts can also easily trigger high network usage. (For example, one broadcast ping might have every host issue a ping reply.)
Picking a subnet size, to handle a specific number of hosts, really has much to do with network usage expectations especially concerning broadcasts. /24 is popular as typically we rarely encounter broadcast related issues on that size network with typical hosts with typical network usage.
Assuming overall broadcast frequency and bandwidth usage is consistent regardless of host's NIC bandwidth, higher NIC bandwidth's with better host processing performance should allow for higher number of hosts per subnet. I.e. newer hosts with "faster" LAN would allow many more hosts per subnet. However, the risk is, this assumes linear expansion, but with broadcast, especially with DoS attacks, broadcasts can trigger exponential network consumption (i.e. broadcast storm).
In your case, real world, if you have 100 Mbps or better switched network, but no L3 switch, I wouldn't be too concerned about 300 modern typical hosts on one L2 broadcast domain. However, if you also had a L3 switch, I would probably keep to the traditional /24.
