I looked at it and it seems like you can only select 35 characters to deploy:
However, you are able to get more than 35 characters in your party. The save that I took those screenshots from has 48 characters in the party. You can only deploy 35 playable characters at a time it seems, but via reinforcements you could get more on the map
All the gray units in this screenshot can’t be deployed because of the 35 unit cap. It looks like it is possible to get more than 35 characters in the party, but you won’t be able to deploy them all at once
