Another Oxford City supporter here
As salmacis mentioned 550 tickets were allocated to St Albans for the NLS play-off when a 3,100 sell-out was expected. I would expect some similar for this game as it will presumably also sellout. However, in the National League, the away allocation was over 1,000 for a couple of games as it was never going to be a sell-out.
The only segregated game this season so far was against Morecambe, who brought around 250 fans. For all segregated games. the away fans have so far been able to buy tickets from the Oxford City website.
You will get the covered terrace behind the far goal plus the open seats on the far side. there will also be some open standing to the other side of the covered terrace. There is quite a decent bar behind the main stand for visiting supporters but the toilets are portaloos.
There is a free car park to the right of the entrance to the ground, whereas parking to the left belongs to the club and they normally charge £5. For the St Albans play-off they turned one of the hard courts into an overflow car park, whilst further parking was in a field on the opposite side of Marsh Lane.
From the city centre there are regular buses to close to the ground.
Hope this helps !