A switch or router will limit the number of clients in a collision domain, thus limiting what can be in the collision domain.

A hub contains a single collision domain and a single broadcast domain, regardless of the number of ports on the hub.

A collision domain is an area on the network where two devices may attempt to transmit at the same time.

A hub has 1 collision domain overall.

A switch has 1 collision domain per interface.

The fewer devices in 1 collision domain, the better.

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Hubs do not reduce collision domains. All devices connected to the hub are in a single collision domain, where as on a switch, each port is its own collision domain.