if it's full you can just park right across from it in the express lot.

You could also book a spot at one of the offsite lots like park and relax— they are very convenient with little if any wait for shuttles and probably less than a mile away.

The BWI website says the current percent open for each of their garages.

Have to say I’m surprised they don’t have an option to pre-book your parking!

Just go to longterm

If you need an alternative -

Two years ago we paid for parking at the Hilton BWI. Free shuttle to the airport, $15 per day parking.

https://www.hilton.com/en/hotels/bwiaphf-hilton-baltimore-bwi-airport/

Hoping to park at the daily garage for several days over the holidays. Will the garage be sold out / full? And is there a way to find out online (before we get to the airport) if the garage is full?