If you want to write transactions containing more than 100 operations, you are required to lock some BTC for exactly 4500 blocks using a time locked transaction.
The formulae to calculate the required lock amount can be calculated using:
requiredLockedBtc = numberOfOpsPerTransactionDesired x estimatedNormalizedFee x normalizedFeeToPerOperationFeeMultiplier x valueTimeLockAmountMultiplier
where the following are constants:
normalizedFeeToPerOperationFeeMultiplier = 0.001
valueTimeLockAmountMultiplier = 60000
For instance, if you would like to write batches of 1000-operation transactions, substituting constant values and estimated normalized fee of 0.000011 you'll get:
1000 x 0.000011 x 0.001 x 60000 = 0.66 BTC
This means you will need to lock 0.66 BTC in order for transactions of 1000 operations to be accepted by ION.