Ideal number of max_num_segments for forcemerge

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Hello.
I'm considering to change the indexing logic a bit.

My situation:

Now my full-indxing batch runs once or twice a month, cause it takes too long to run more frequently(definately it needs to be refactored).
Holding the new index till the full batch ends then turn it live by changing alias.
Then partial indexer runs every secs by users' actions.

I want to improve a full batch by disabling refresh_interval cause it doesn't need to be refreshed while bulk indexing.
As I read this document, I need to call forcemerge after all.

Planning:

disabling refresh -> full batch -> forcemerge (I guess?) -> change alias -> partial indexing by user actions

My question is:

  1. In this case, what number should i set for max_num_segments?
    The index is around 30GB sized with 2 shards 5 replicas 20 nodes (containing other indices).

  2. Is refresh=false default for bulk-indexing if it is the newly created index?

  3. Do I need not to add disabling refresh or forcemerge process in this case?

I've tried to find any formula or information for this, but I couldn't find which could be the factor to decide the number.

Thank you in advance.