Read IBs description on this here.
https://www.interactivebrokers.com/en/index.php?f=14193&ns=T#market-data-fees
The number of symbols that can be viewed simultaneously via the TWS deep book windows (including BookTrader, Market Depth and ISW) is determined as follows: one unique symbol for every 100 allowed lines of market data, with a minimum of three and a maximum of 60.
Basically you get 3 L2 data sources at any one time, and you can get more based on account value (1/$M, so $4M+ helps), trading a lot (over $2400 in prior monthly commissions, each $800 gets 1 more), or paying $30 each/month for quote boosters.
In short, you’ll never get lots of L2 data, say for API use, but if you’re active or you’re willing to pay a bit for it until you are, you can get a few more live L2 windows. I would not have more L2 windows open than you have data permissions for - it can get weird about which one isn’t getting live market data.