which will be the impact of a vlan adds with capacity of 1024 host on a
switch where the process in an average of 20 % on a switch WS-C4507R?, the
traffic of broadcast affect the process of the switch?.
I would suggest making some practical measurements on the real hardware if you have the possiblity.
1. Configure the switch as a DHCP relay agent.
2. Start some workstations issuing DHCP requests.
3. If you have a Cisco device as a DHCP server, then use the "debug ip dhcp server packets" to see the DHCP server receiving and sendig the DHCP packets.
4. On the DHCP relay agent (the switch) use the "show processes cpu sorted 5sec" command to display the burden on the processor in a 5 seconds interval.
5. From the increase of cpu load compared to the DHCP inactivity period, you will be able to make extrapolation on what can be the load increase with 1024 hosts.
Of course, take into account that not all workstations will be booting at the same time when people come to work in the morning, but there can be a time interval where DHCP requests will be spread across.