What is the private IP that I may use if I am about to subnet a total of 743 hosts in my office? Should I use 10.x.x.x, 172.16.0.0 - 172.31.x.x, 192.168.x.x or all of them are allowed? Also if more than one is allowed, which one is the best for my condition (743 hosts). Thank you.
All three blocks are private IP ranges, aka. they are not routable on the Internet. Which one you pick is of little significance, there's no technical difference between them.
However, you don't need such large blocks for your amount of hosts, so you should definitely subnet whatever block you choose.
For 743 hosts, you would need a /22 block, which contains 1022 available host addresses, giving you ample room for any potential growth and other infrastructure.
So you can pick whatever you like, for example 10.22.116.0/22 or 192.168.60.0/22 or something else entirely. Just make sure that whatever range you pick doesn't overlap with something else in your network.
Just to inform you, you can also use IP addresses in a way that will provide you more information like;
10.x.y.z/24 where x can be a building y can be a floor. 10.1.3.0/24 will correspond to the first building third floor etc...
If this is a fresh install and your equipment allows you, consider subnetting into smaller networks like 3 x /24 rather than single /22 networks.
Last but not least, if this is a fresh office, I hope you also consider other devices that will use IP addresses apart from your clients. wireless, Printers, network devices(management IP addresses), Ip telephony (if you have any), card readers, security cams ... etc. You may want to segregate printers to a different subnet and make them talk only with the print server, or security card readers and cameras should be on a different IP subnet.