Typically you are able to buy 1-3 CRTs in a given tier during a single transaction, although some Issuers may allow for a larger number of CRTs in a particular tier.
If you want to buy more, just repeat the process. Please note that in order to submit additional transactions in a given offering, you will need to wait until your existing transaction clears, i.e. goes from a "Pending" status, to "Payment Received".
How many CRTs can I buy?
