Is there a limit to plotting markers with folium?

As @Bob Haffner suggested you can use FastMarkerCluster from Folium library. Here is my code, in my file there is ~500K points.

import pandas as pd
import json
from folium.plugins import FastMarkerCluster

rome_lat, rome_lng = 41.9028, 12.4964
with open("file_name.json", 'r') as f:
  # create a new DataFrame
  samples = pd.DataFrame(json.loads(f.read()))        
# init the folium map object
my_map = folium.Map(location=[rome_lat, rome_lng], zoom_start=5)
# add all the point from the file to the map object using FastMarkerCluster
my_map.add_child(FastMarkerCluster(samples[['latitude', 'longitude']].values.tolist()))
# save the map 
my_map.save("save_file.html")

This code takes ~10ms to render the map.

For more details example please follow this link: FastMarkerCluster example

Hope this is helpfull...