First

Depends on the pot size and strain! You could probably get away with 4-6 easily depending on pot size! I plop autos in 2 gallon pots all the time—- that would allow about 8! Make sure you have good light and airflow! ✌️🏻💚🌱

The flower stuff you've hopefully already purchased.. something p-k like.. Plant still needs micros so don't cut out the micro nutrients but the higher P-K should be fed.... And love the nutrient Love is super important dare I say it's more important then nitrogen (it's not). :)

This is very common with P and K toxicity. Excess P and K causes the lockout of micronutrients and calmag. Now, you have to decide how you want to treat that. Your plants just finished stretching, so now is the time that they will be increasing their intake of P and K by a lot. So you can try to fix the problem by giving her a feed of pH'd water only. You could also not change a thing because it's possible that your excess P and K are going to be necessary now that she has finished stretching so the problem may fix itself.