"What are the rules that decide how many are included or...

"What are the rules that decide how many are included or excluded during each rebalance?"

I wrote the following about another stock and the ASX 100, but S&P probably adopt a similar approach with the ASX50.
I am not sure that it is possible to reverse engineer S&P's process in order to predict which stocks will be included and evicted. I expect some folk give it their best shot. Here is a link to S&P's methodology.
https://www.spglobal.com/spdji/en/documents/methodologies/methodology-sp-asx-australian-indices.pdf

Inter alia, in order to limit turnover of the stocks in the indices, S&P have separate hurdles for both admission and eviction.

For the ASX 100, the stock has to achieve an adjusted capitalisation ranking of 84 or lower for admission and 116 or higher for eviction.

The adjusted capitalisation ranking depends on the free float of notionally tradable shares as measured by S&P's Investable Weight Factor (IWF).

S&P deems the shares held by directors to be in the category of shares that they consider to be non-tradable.

Thus, the relatively high proportion of shares still held by PME's founders handicaps PME's IWF and hence its capitalisation ranking.
Nonetheless, its pleasing to see that PME is knocking on the door to the ASX100.

Cheers