I'll be in the Himalayas for 3 weeks at high altitude but NOT climbing. I plan to take acetazolamide 125mg twice a day 2 days before/after the trip. Should I take the pills EVERYDAY during the trip? Thank you

Acetazolamide should be prescribed by your physician, as it can interact with other medications and medical conditions. Also only taking it before and after your time at altitude sounds a bit iffy. You must check your plan with an expert since altitude sickness can be life-threatening. Will you be acclimating at a lesser high altitude before your time in the mountains?

Clearly you should only take acetazolamide as instructed by your doctor.

The normal advice is to start taking it 1 to 2 days before ascending and to keep taking it while you are ascending and for at least 48 hours after you have reached your final altitude.
If you develop symptom of severe altitude sickness, it is essential that you seek medical advice and descend to a lower altitude as quickly as possible.

The only real way to avoid altitude sickness is to ascend slowly . See https://www.nhs.uk/conditions/altitude-sickness/ for more information on how to recognise and deal with AMS.

There is no real way of know how you may be affected by altitude though it is safe tis ay you will be affected to an extent. Everyone is affected differently. Arriving in Cusco , Peru 2 3400m I was quite badly affected, severe headaches, nausea etc. My wife barely noticed. I have been at altitude many times since and now am not barely affected. Whther this is because I am getting older or just adapting, I dont know.

I have take acetazolamide once when we had to go direct to 3800m from sea level. We were trekking up to 5000m within a couple of days. It was still tough but no AMS.

There is plenty of advice around about how to deal with altitude some good, some total rubbish. Drugs will help alleviate the symptoms but wont prevent AMS. Ascend as gradually as possible, take it easy for the first few days, no alcohol, keep hydrated, eat sensibly etc...