is there a maximum number of songs allowed in a play list?

This is what I saw...

The backstory; I got 13,000 songs in my iTunes. Using Garmin Express to manage my music, I selected a playlist that had 1,100 songs (about 6 gig). Seemed easier to pick that playlist, instead of manually copying over each song.

Every time I did that, the watch was no longer recognized by the computer or GarminExpress. It was the first thing I added, so there was plenty of room left in the watch's memory. When I'd selected the playlist on the watch, it would hang. Finally had to do a factory reset. I repeated this process 3 times.

Now; I broke up the playlist into 10 playlists, each with about 100 songs. Copied all 10 to the watch. Everything is fine. Copied some other playlists, making the song count beyond the initial 1,100. Everything is fine.

Conclusion; Forerunner 965 does NOT like REALLY BIG playlists.

Same problem here, I just reported this problem today and they opened a ticket to review the issue. 

In my case, this is what I experienced after loading up several MP4a playlists straight into my watch via Garmin Express:

1.  Watch slowed down a lot to the point that I had to restart it.  Once it came up transitioning between menus was slow (it seems that the watch was still loading up something in the back).

2. The watch froze when I selected a playlist with 500+ songs and it took about 5 minutes for it to let me play a song, in the meantime, I am sitting there when I was supposed to be running using my Forerunner!!!

3. When I tried to play another song from another playlist....more freezing so I just decided to use my phone which is already connected to my headphones (old way).  I was working out in a bad mood, not good for sure.

It looks like that this watch acts fast if the playlists are "relatively" small (how many songs?), but when you have a big one forget it, it will make you miserable.

To make matters worse, I connected the watch to Garmin Express and it took 10 minutes for the watch to connect with the computer, which was offering me a maps software update that I did not take...but when I disconnected the watch it said that it was updating the maps!!!  It never came back to operation and had to restart it again.

The point here is simple, Forerunner 965 is supposed to be a PREMIUM watch but is not acting like one.  If the user has to be praying for reliability every time a playlist is chosen for a run, hike, etc...it would probably be better to choose a different alternative.  How do we know if the sensors reliability is not compromised by whatever is going on inside of this thing?

This is my 3rd Forerunner and I am pissed....spent 1/2 of my free Friday dealing with this stupid thing...

Let's see what they say but I am not happy at all.

P.S.  I still have 10 more GB to use for extra music...I know right?

I have a playlist with 1096 songs....
I guess your desire to have a personal playlist with such a huge number of songs is doomed to failure.
Garmin certainly needs to be careful about the wishes of users. And to any appeal, even insignificant, according to the manufacturer, to answer and give recommendations or an official answer. What we do not always see in this forum. If you make an official appeal to support, you will definitely get an answer.
Earlier there was information, although it refers to the limitation of songs in the playlist for Deezer, where the recommended number is limited to 100 songs.

support.garmin.com/.../

I also had experience with the conversion of a playlist of 250 MP3 songs, 2.5 GB in size, the clock opened them with great difficulty, sometimes I had to wait 20-30 seconds.
Now I use only compiled streaming playlists up to 100 songs.