You can buy a single or multiple ETFs at the same time.
To add new ETFs to your portfolio follow the steps below;
Open your portfolio> Add securities> select the ETFs you want to buy> Add to portfolio> Continue> Set your portfolio weights > Review and continue> Save and continue.
Once you have selected the ETFs you want to add to your portfolio, adjust the weightings of your portfolio then fund and rebalance your account. The new holdings will be bought during the next trading window.
You can also add ETFs by going into the Options tab in your portfolio, see the below;
Open your portfolio> Options> Edit portfolio> Add securities>Select the ETFs you want to buy> Review and continue> Save and continue.
The new weightings are not final until you have run the rebalance.
If you want to simply buy more of the ETFs you already currently hold based on your portfolio's weightings. you can do so by following the steps outlined below;
Open your portfolio> Invest> Enter monetary value you want to invest> Review order> Submit
To buy a single ETF, you add them to your portfolio with a 0% weighting and then fund them individually. You can do this with as many ETFs as you like. If you purchase ETFs in this way, if you ever decide to sell your portfolio as a whole, the system will also sell these 0% ETFs holdings as part of the whole portfolio.