The number of accounts you can connect for each of your integrated apps depends on your plan. You can connect two accounts from the same app by default but this number will increase with higher-tier plans.
The following image breaks down exactly what is offered at each plan tier. As you can see, the professional plan allows for 6 accounts per app.
Note: Prices shown are for annual billing. When billed month-to-month, the Starter plan is $4.99 per month, the Personal plan is $9.99 per month, and the Professional plan is $14.99 per month.