My 93-year-old mom passed away in May and we are having her memorial service this coming weekend. Is it typical to not know in advance how many people will attend?

That is normal. The funeral for my mom was open to all comers. I knew pretty well how many family would attend, most had to travel and let me know their plans. There was a large attendance from her church, where the service was held, and some from the senior center, and some other friends and neighbors. I could not have predicted the total, some attendees surprised me, some others I was surprised not to see.

I had no idea who or how many would attend either of my parent’s services. It didn’t seem important to me then or now. It was a day I simply wanted to get through. I hope you’ll get what you’re looking for and wish you peace

It happens, especially in the summer. You never know who is out of town etc. Sometimes more people show up than you expected and sometimes less. Usually a respectable amount show up, at least that has been my experience lately. (Been to a lot of funerals these days)

I think this happens with memorial services, and its ok as long as you are not catering substantial food/ meals that costs you a certain amount per person, so you may be at financial risk.
If its a memorial service with open attendance hours during a few hour window, eg at church, I think its pretty common to not have RSVP requirements and not know how many people are coming, and there is no harm in that. In that situation there is no need to actually know the exact number.

As far as the emotional side of not knowing how many people will attend, or perhaps are you worrying that only a few people will show up? Hopefully you wont worry about that, try not to. Keep it in mind that you and your sons are going to honor your parents, and that's the most important thing. Try not to worry whether or not a lot of people come. Hopefully you will be pleasantly surprised with a good attendance.

Is what odd? That you're traveling over 300 miles to attend your moms memorial service? No that's not odd.
Or because she died in May and now it's August before you're doing something? Again, no that's not odd. Lots of folks especially since Covid have opted to delay funerals/memorial services.
Or are you asking if it's odd if no one else attends but you and your boys? To which I will once again say no, it's not odd as you are the most important people that should be there anyway.
This service is more for you and your boys than anyone else, so I hope you just enjoy it however it turns out.

It is normal to be wondering who will attend a service like this.
You are doing what you wanted to do, honoring what your mom wanted. That is the important thing. If it is just you and your sons you can still make the most of the trip. If a few or many show up enjoy the visit with them. Have a nice visit, share stories and memories.

When I arranged the funeral for my Husband I had no idea how many would show up. I chose to have the wake and funeral the same day.
I was floored as to how many showed up. the Funeral director said to me that I probably should have had this scheduled for 2 days. I told her that I had not seen some of these people in at least 10 years. (where were they all when he could have enjoyed some of the company early on in his dementia?!)