There is no limit stated in the terms and conditions.
https://www.eight.com.sg/terms-conditions/
eight BASH! Top up $28, get $28 FREE
This promotion is valid from 21 May – 24 May and only available at the eight BASH @ CityLink Mall or at any of our Service Centres.
A valid and successful top-up transaction is defined as a minimum of $28 top-up amount. For every successful transaction, $28 additional credits will be credited to the customer’s wallet balance.
Additional credits will be credited to the mobile service’s main wallet balance within 7 working days from the date of the successful top-up transaction.
Customers are entitled to a maximum of $84 i.e. 3 transactions in additional credits per mobile service throughout the duration of the promotion.
This promotion is not transferable, exchangeable for cash, or redeemable for other products/discounts.
In the event of any dispute, the decision of eight shall be final.
All eight’s Mobile Terms & Conditions apply. eight reserves the right to amend, withdraw, or terminate this plan, promotion and its Terms & Conditions at any time without prior notice.