An easy way to get follower counts for multiple users in a single call would be helpful, but until something like that gets added (if at all), there are ways to help with running into API rate limits. For example, using an app access token to raise your limit to 120/min, or if there’s sufficient reason for it you could submit a request to have your limit raised above that.
Another option to think about would be to make use of webhooks, you could make a request to the follower endpoint to get a number, then use the webhook to keep it up to date (with some degree of error due to unfollows not being notified), and maybe if that is accurate enough you will not have to poll the follower endpoint as frequently.