The likelihood is that we will be getting the same allocation as for the league match, namely 1,980.
On the day after Forest were confirmed to be in the play-offs (ie last Wednesday), their club told their season ticket holders that sales were open immediately and that they could buy their play-off ticket which be loaded on to their season ticket cards in the usual way - just like for a cup match.
Forest did this immediately before even knowing which day they would be playing their home leg, or who they would be playing. Appears that they have handled it rather better than us.
The fact that season ticket holders have been able to buy their own seats would tend to suggest that the arrangements at the City Ground will be precisely as they have been all season for league matches.