You are not necessarily limited by the number of forwarders per indexer, but rather by the amount of data your indexer can ingest. If your server for example is capable of indexing at ~70 MB/minute, and your indexers send 0.5 MB/minute, you can expect roughly 140 forwarders per indexer. It is however likely that your forwarders will send less data than that so the number of forwarders will likely be greater.
In the end it really comes down to the hardware of your indexer. I recommend taking a look at Hardware capacity planning for your Splunk deployment in the docs and estimating the amount of data you want to index to get an idea of how many indexers you will need to support your forwarders.
