Getting Number Of Assets Of Specific Account

Currently I'm trying to get Number of Assets of Account like in SolanaFM Exporer (like here: https://solana.fm/address/99yEb8Urs2SR5Y6xMQJTWkWZSkXXLp5ssNjcqvHrV6MB/transactions?cluster=mainnet-alpha)

I didn't find any endpoint that could help me with that on SolanaFM API, also how I checked even SolanaFM exporer itself doesn't use it's own API for this task.

So how can I get Number of Assets by standart RPC of maybe any other API?