how can i send unicast transmissions to more than 10 devices, one cordinator

What product? Smart Energy or standard ZigBee? Are you using routers or end devices? Are you have an issue joining >10 devices? Or just reliably communicating once they are joined?

For large networks you may need to use Source Routing otherwise each transmission will require full address discovery which is very inefficient and could be a problem you’re running into.

Your gateway and/or XBee manual will have more detail on how to enable and use Source Routing.

i use xbeepro S2B i want to do network between 4 xbees
one cordinator and 3 router or end devices. unicast mode use when i have two xbees if i want to use more than two then i should be in broadcast mode but when i sent message it sent to all xbees .i dont want it .i want to select device which i want to sent data to it

You can definitely unicast to multiple devices. Just send the same API frame down to the XBee with a different address each time (possibly with source routing depending on network size). How are you sending the message?