Liquid cooling fittings are always have got miscalculated. The type and quantity of connectors depend on tubing size and blocks, radiators, pumps and reservoirs layout. It is always recommended to have a few extra fittings just in case something doesn't work as expected.
By a construction fittings can be split in two major groups: compression fittings and barbs. Barbs are less expensive, but it is recommended to secure tubing and barb assembly with clamp to make sure those two won't disconnect by accident. Compression fitting on other hand do not require clamps as they use compression locking mechanism to hold tube tight and secure.
Angled fitting and rotary adapters help to avoid tube excessive bending and sometimes it is the only way for safe route tubing in fancy water cooling setup.