How many open markets on one exchange is too many?
Do you plan to support binance?
that usually depends on the exchange : you must not overflow the API Call rate limit.
You can manage the "Exchange Preferencies" options to set the correct delay.
If you have a lot of open markets and your program is slow you can re-run cat using more ram (there's also an option about that)
About binance : i can support that exchange but i must work with exchange admin during interface phase.