Not familiar with the game yet
I'm wondering : is there a lot of room to play vs AI (number of scenarios, etc.) or is it mainly a MP game, where I'll have to search for opponents ?
The game has three campaigns, with multiple missions each. Those are a couple of hours of content, but probably don't make the game a worthwhile purchase if you are looking for a "story-driven" single player experience.
The game does, however, allow you to play skirmish matches with 1-7 AI players on all the maps and in all game modes that are available in multiplayer, except 10v10 which is limited to dedicated servers (and therefore, PvP multiplayer). This does include the recently added game mode "Breakthrough", which offers an asymmetrical 1v1 match (attacker vs defender) on multiple maps.
The AI is fairly competent and should be able to entertain you for a decent amount of time.