From my experience as a player and as a DM, rolling lots of checks usually plays against the PCs.
If you're trying to sneak past 2 guards you have to roll twice. If your sneaking bonus is high enough, you can do it. But if instead of 2 guards, we have 10, you'd have to roll 10 times separately, knowing that failing just one of these checks results in you being discovered and the guards raising the alarm.
For this reason, I'd set a DC depending on STATIC variables (depending on the surface you're walking on, if you're carrying a light, if there's fog or it's dark, or whatever), then impose a penalty to the PC's roll (equal to 1/2 the number of guards for average alerted guards, 1/4 for distracted guards, o 1/1 for guards on alert)
Halfling sneaking past 2 scoundrels, has to roll ONCE against average enemy Luck check (usually +0), plus 1 (1/2 of 2 scoundrels, which are not distracted but not yet in alert).
This reduces the time wasted at the table rolling dice "until the PC fails", requiring just 5 sec more prep from the DM
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