The first thing that breaks is the sub-tax calendar, and it breaks in the second state rather than the second county. Two counties in one state share a statute, so you're tracking different dates against one rule set. Add a state and you're tracking different dates against different rules for how long the redemption period runs, whether subs earn the statutory rate, and how much notice you owe before you can move on a lien. Missing a sub deadline in some states subordinates you to a later holder, and in some it costs you the right to foreclose at all, which is a state-specific determination worth having an attorney there confirm in writing once rather than guessing at eighty times.
The second thing is notice. At a dozen positions almost nothing reaches the end of its redemption period. At eighty, a handful will, and the notice process is the procedurally exacting part of the whole strategy, every party with a recorded interest, service in the manner the statute requires, proof filed. That work arrives on the statute's schedule and not yours, and it tends to arrive during the next auction season.
Spreadsheets survive longer than people expect if the sheet is one row per certificate with the parcel, county, purchase date, redemption deadline, next sub date and dollars in. What kills the sheet is the second person touching it and the moment reconciliation starts, since interest accrues per certificate at a rate that may differ per certificate and your accountant wants basis per position for tax purposes. That's a conversation with your own tax professional, since treatment of interest and of capitalized subs isn't uniform.
Budget cash for subs as a hard reserve rather than a forecast. Being fully deployed at auction and short on a sub payment is the ordinary way people at your size lose a position they'd already paid for.
