You can save as many times as you want. However, if you saved multiple times in the course of a day, your balance will be merged into one. Subsequent saving attempts on other days will be treated differently.
What this means is that if you save N1.000 x 5 times on Day 1, it is merged into one transaction as N5000 and the 7% interest begins to accrue on it for 365 days, however, if you save another N2,000 on Day 2, it will be treated as a separate transaction and the same 7% interest begins to accrue on it for 365 days, however, the maturity dates for both investments will be different.