I don't think you will get this from FPR datasheets as they don't use routes as metric for firepowers. They are more oriented towards connections instead of routes.
Having that said, you can query your internal team to see if they can provide that number. Its subject to memory size and if you are looking for large scale routing my recommendation is to provision separate ASR routers to take care of that. Let ASA focus on security part and use summary routes or stub routes to avoid routing overhead.
