Manifold selection is usually made by copying the last installation. That works until the application changes, at which point an under-specified manifold makes calibration awkward and an over-specified one wastes money and space.
A two-valve manifold is an isolate valve and a vent valve in one body, used with pressure gauges and single-input pressure transmitters. The isolate valve shuts the process off; the vent releases trapped pressure so the instrument can be removed safely. If your instrument has one process connection, this is the manifold you need.
A three-valve manifold adds an equalise valve to two isolates, for differential-pressure transmitters. The equalise valve is the important one: opening it before closing the isolates puts the same pressure on both sides of the transmitter diaphragm, protecting it from the one-sided overload that would otherwise occur during shutdown. Sequence matters - equalise first, then isolate.
A five-valve manifold carries two isolates, two vents and an equalise. The additional vents allow the transmitter to be zeroed and calibrated in place without breaking the process connection. On installations where calibration is frequent, or where access is difficult enough that removing the instrument is a half-day job, the five-valve pays for itself quickly.
So: a single process connection with infrequent service points to a two-valve; differential pressure in standard service to a three-valve; and differential pressure with in-place calibration or difficult access to a five-valve.
Direct-mounting the manifold to the transmitter removes the impulse tubing between them, which removes leak paths, mass and installation labour. Where the transmitter must sit remote from the tapping, a pre-fabricated hook-up achieves the same reduction in joints across the whole run.