1) Which integration method will be used when data in onboarded with following steps
a) HEC method
c) DB connect ?
2) How many API scripts that we can able to run in HF?
If possible can you please suggest any documentation and also the uses for the above methods individually?
about the first question:
"integration" is a great word with a large set of meanings, what do you mean with "integration"?
Probably you don't mean "ingesting" because you already have your ingestion methods.
Maybe you mean "parsing", in this case you have to use the same methods of each ingestion method in Splunk: you have to use a Technical Add-On to correctly parse your logs.
About the second question, there isn't any limit in the number of concurrent APIs in your HFs, they depend only on the resources available on the HF and there isn't a casistic or a benchmark.
My hint is to try with your APIs using a standard configuration for the HFs (12 CPUs and 12 GB RAM), then you could monitor your HFs to understand if the resources are sufficient or if you need to add more of them to your HFs.