ThedoWorkmethod inserts an account for every iteration of theforloop, which runs fromi = 0toi <= 200, inclusive.
This results in 201 iterations, and hence 201 accounts are inserted into the org.
Explanation of Code Behavior:
The loop conditioni <= 200runs 201 times (0 to 200).
Theinsert acct;statement is executed 201 times.
Apex Developer Guide - Loop Constructs