From the Guardian article, in the 12:52 pm update, (archive):
Two others arrested at the scene with guns are being investigated. A fourth person arrested may have had nothing to do with the attack, according to New Zealand’s police commission Mike Bush.
Several guns have been recovered from both mosques. Two explosive devices were found on two vehicles at the scene. One has been disabled.
He only had the one car. Didn't see explosives in it, only gas canisters.
Even if they counted his car, that still leaves at least one more car with explosives, plus guns stashed at both mosques.