Heroku Sidekiq: Redis connection either too low or max number of clients reached

Question

I am running sidekiq workers task and using redis to go nano on heroku.

My sidekiq.yml looks like this:

Sidekiq.configure_client do |config|
    config.redis = {size: 1, db: 0, url: ENV["REDISTOGO_URL"] || 'redis://localhost:6379/0'}
end

Sidekiq.configure_server do |config|
    config.redis = {size: 3, db: 0, url: ENV["REDISTOGO_URL"] || 'redis://localhost:6379/0'}
end

When I am setting the size for server less than 7, it gives me the error :

Your redis connection pool is too low. Must be atleast 7 connection.

When I am setting it to 7, it's giving me

max number of clients reached.

I don't understand why is this error arising and how to solve this problem for heroku. What is the optimal number where neither error happens ?