Base capacity is 2 simultaneous sites. Each Construction Company adds +2 sites (Robotic/Autonomous versions add more). If the queue is full, new buildings wait their turn.
Economy tab → Factories & Buildings → Infrastructure & Logistics → under the Construction heading. Each Construction Company adds +2 simultaneous sites and daily construction power; Robotic/Autonomous versions give even more. If your build queue is always full, this should be your first investment.
A daily construction power pool is shared among sites: construction-sector buildings add power, and some policies and difficulty perks speed it up too. Big projects (like a nuclear plant) naturally take months.
Orders are processed in sequence; from the queue window you can cancel or change priority. During construction, materials such as cement and steel are delivered from your stock.