Currently about 15% of the US refining capacity is affected.
Remember this, for discussion's sake, let's say all Shell and BP gas stations closed because of the strikes, all the other brands would have to take up additional load.
All the independent gas companies who get their gas from BP, might have to get it from somewhere else.
On strike is not the same as shut down. Only one refinery in California shut down so far. They already were half down due to maintenance/upgrade work.