[ledger] reduce the number of instances in the convergence test

This reverts commit 99311885
"[ledger] Re-enable full range of ledgers for convergence tests."
- it seems that the higher number of instances makes the test crash
flakily.

Reducing the number of instances while we investigate.

LE-752

Change-Id: I289a39e4afc9e493624f08302872b3155c27f3ac