How many attempts do I get for OTP verification while downloading GST reconciliation? How do I configure einvoices in BUSY?

There is no such attempt limit for OTP verification while downloading GST renciliation data as you can reattempt if OTP is not received within 1 - 2 minutes timeframe.
To configure and generate e invoice in BUSY you have to follow below steps:
Step 1: First you have to create a GSP (GST Suvidha Provider), you need to visit the eway bill or e invoice portal and select Webtel Electrosoft Private Limited and Pinnacle Finserv Advisors Private Limited as the GSP Vendor type.
Step 2: Go to Administration and click on configuration.
Step 3: Select Features / Options and click on GST / VAT, here you will see a option called E-Way Bill / E-Invoice Required.
Step 4: Set Y to turn it on and mention your place and pincode and GSP username and password in GSP Configuration tab and save the configuration.
Step 5: Go back to Administration and click on configuration.
Step 6: Choose the Voucher Series Configuration option and select the voucher type for which you want to generate eway bill and e invoice, such as sale, sales return etc. Then, select the voucher series and click on Voucher Configuration.
Step 7: Here, you will see options of Input Transport Details, Generate E-Way Bill / E-Invoice After Saving , set Y to enable that. Now your BUSY is ready to generate E-Invoice automatically.