Any network can be hacked, but if you configure the vlans correctly and do MAC address filtering (i.e., assign each devices MAC address to a specific port), then you would be fine.
If you want to add additional protection and ease of maintenance, you could use
PacketFence which is an open source Network Access Control (NAC) package. It uses SNMP traps to monitor and control the switch ports.
We have 35 locations and each location has 30+ switches (managed by PacketFence) with multiple VLANS and have not had any security issues.
Regarding the choice between using one 48 port switch vs two 24 port switches, I'd go with two 24 port switches. The cost difference isn't that much assuming you're comparing the same brand and class of switches and the 2 switches add more flexibility.