Simply mathematics suggests that yes, 100 IP addresses is sufficient for 65 hosts.
However, what would be more telling, would be the growth trends you've experienced so far: how quickly did it get to 65 hosts? Do you expect this trend to continue? That's a 5 minute email conversation you can have with HR and it'll help immensely with your planning.
Another caveat would be wireless clients: how many wireless leases do you have/expect? Often people will bridge their wireless network onto their existing network, keeping the default lease duration of 7 days. However, if you have a steady flow of smart phones, tablets, laptops that may or may not be there the next day (i.e. guests, like clients, contractors, etc.) then you can quickly burn through your available pool if the clients are not releasing their addresses and notifying the DHCP server.
Generally speaking, I find /24s to be the most useful for everything but iSCSI/storage networks, DMZs and "glue" networks between routers (I use /29s, enough for the routers and a laptop if I need to sit on the same subnet with a laptop): its the subnet everyone knows, it's a happy size for broadcast traffic, and once you get beyond that, you're usually breaking up your network into more manageable chunks with VLANs anyways.