Ventilation is important for health and comfort and that's why the building regulations require windows and doors to provide a suitable amount of ventilation to rooms within a dwelling.
The type and amount of ventilation will be dependent on the use and size of the room.
For example, rooms where steam will be produced (kitchens, bathrooms, utility rooms etc.) should be provided with higher levels of ventilation (normally mechanical fans and windows) than other rooms where suitably sized window openings and background trickle or night ventilators may be enough.
Any replacement windows should not reduce the area that can be opened for ventilation.
Should I have a night vent/trickle vent?
Background ventilation is the flow of external air into a home that can be provided without opening a door or window, and the vents are simply slots in a window through which the air can flow. These are manually controlled and can be closed if external weather conditions warrant it.
They aren't required if the room already has an adjustable airbrick.
A replacement window also has to have vents if the window it replaces was fitted with them, and this is good practice anyway to prevent condensation.
You can also have a night vent where the window is lockable in a slightly open position although this isn't recommended as it could be less secure with an increased risk of draughts.