If you need more than 300,000 conversions per month, the best way is to get a second OCR subscription, and then use both API keys in your API calls. This will give you 600,000 conversions total (or 900,000 for 3 subscriptions/API keys, etc).
For more details please see this ocr forum post.
The same method (= using more than one api key) also doubles/triples/etc your rate limit 
Important: Starting December we will allow 100 conversions/minute in the regular PRO plan at no additional cost. So having two plans/key will then give you 200 conversions/minute or using three api keys will give you 300 conversions/minute (etc).
This rate limit upgrade will be automatically available for all users, there is nothing special to do from your side.
We use a rate limit to encourage users to distribute their ocr load a bit more evenly over time, if their application allows. This allows us to keep the cost per conversions down for everyone. Currently we have the - by far - lowest cost per conversion in the industry.