It is actually almost impossible to predict how many queries you will receive without knowing not only what IPs your customers are coming from but also the network configuration for each one of your customers. Since this is not possible you can rough (and worst case) number by this calculation.
(Take the average seconds the average user stays on your website) / (TTL of your record)
Total number of sessions per day
Multiple this by the number days in the month
Add one for each email received.
The average user stays on your site for 10 minutes.
Your record TTL is 5 minutes.
So the average user will cause you 2 queries per visit. (10 / 5 = 2)
You have 10,000 visits per day (2 * 10,000 = 20,000 queries per day)
You have 31 days in the month (20,000 * 31 = 620,000 queries per month)
So you would have 620,000 queries in the month from web traffic.
Take number and add it to the total number of emails received.
So if you received 10,000 emails then your total queries would be 630,000.
This is of course a rough estimate and it is usually a worst case number. Since many users would share the same resolving name server then they actually share the same query.