Do you mean practice squads? Because I find it hard to believe enough QBs currently on NFL rosters will be let go between now and January AND sign with the XFL to fill out multiple starting QBs jobs in the league. Even coming from practice squads might be a stretch. NFL QBs drop more and more the deeper you get into the season, which means more and more backup-types/starting-caliber XFL QBs will at least get signed to be third-stringers on the practice squad.
From where I sit, these are the places XFL QBs will come from:
-Obviously, those working with Jordan Palmer.
-Players who've already been invited to the draft pool - XFL backup or third-string types who don't even rise to the level of qualifying to work with Palmer. In other words, those off the radar who perhaps impressed at the Showcases, or come from the IFL or other, smaller leagues. Similar to non-assigned QBs in 2019 XFL Draft.
-QBs who are currently unemployed but going through workouts with NFL teams during the season. Perhaps as we get closer to the winter, if they are still out of work, they may opt for the XFL and become available in a supplemental draft in Dec or Jan.
-QBs from the USFL who choose to cross over (Bryan Scott?) who won't be eligible until the first of the year (potential Jan. supplemental draft)
-Those currently on NFL practice squads who aren't signed to futures deals after the NFL season or who spurn futures deals to sign with the XFL.
The highest-quality QBs will likely arrive the latest - Dec. or Jan. It will be interesting to see how easy/difficult it will be for them to win jobs in training camp if they join the league very late.