6th and 7th, according to the various Space Marine-centric Forgeworld books through the editions.
War Machines of the Adeptus Astartes:
"The crews for a Chapter's armoured vehicles composes solely Space Marines, many assigned as required from the Tactical squads of the Chapter's 6th and 7th Companies. All Space Marines are trained in the operation of armoured vehicles and armoured formation tactics as part of their core indoctrination - some going on to specialise in armoured operations, being trained upon larger vehicles and even being entrusted with basic maintenance tasks."
That said, according to the Codex itself, drivers, pilots, and gunners are typically Techmarines:
page 85:
Techmarine Pilots:
A Chapter may have any number of Techmarines who exist outside the company organisation, belonging instead to the Armoury. Most Space Marines receive vehicle training during their time in the Reserve Companies to ensure a degree of flexibility, and so are capable of acting as drivers, pilots and gunners themselves, but such positions are typically held by Techmarines. Once a Techmarine is interfaced with a vehicle's machine spirit while performing these duties, the meld will occupy his full attention - though he cannot effect repairs, his bond with the vehicle makes it all the deadlier in offence.
(Also, the old Death From the Skies book stated that, in almost all cases, it is Techmarines who pilot aircraft & gunships).
Presumably, this is why aircraft pilots are always shown with the full red outfit, whereas vehicle gunners are often shown with only one red arm, or no red on their armour at all.