I’m sure the actual number of agents is higher than most of us could imagine... But accounting for 90% of all DHS employees would be staggering.
DHS does not (yet) control or manage all of the armed agents.
The overall percentage of armed agents runs from 5% to 45% of total staff for those Agencies that are authorized to have them.
Note also, that DHS and other Agencies employee a significant number of private contractors as "Armed Civilian Contractors". Look for this number to increase.
Contracts with armed private security firms are invariably redacted and obscured with every bureaucratic process available, but it is clear that there are more of them every year, and the dollar volumes appear to be increasing.