Maximum number of objects in flex directory

Not sure what you mean by objects in flex directory, but here's a quote from their blog post on 1.7 RC1 about flex pages

Traditionally we recommended less than 1000 pages for Grav sites. That's not a hard limit, and does depend on plugins you use, and logic you may have in Twig etc. Some people have run Grav up to 10,000 pages with extensive caching and other performance optimizations. With Flex Pages we have tested Grav sites with over 50,000 pages! The admin was essentially non-functional with this amount of content in Grav 1.6, but in 1.7 and with Flex Pages it was totally usable.