Right now we trade only the NQ's and the Dax. Between the two markets there is plenty of room for us to grow. We are a tiny drop in the bucket in both markets. We never trade more than a couple hundred NQ's per day. Even so, to avoid slippage we don't just blast our trades into the market with one order. If we want to use a market order we'll spread a few trades out over a few minutes. If we want to use a stop order, we'll use a few different price levels.
You can easily see how much you can trade at a time by looking at the size of the bid or the ask. Here's a good exercise for you...Around the time you usually trade take some data on how many contracts are offered at the bid or the ask. Check like once every minute for 50 minutes, or so. Do this on an average volume day (or across several days). Now take the average and the standard deviation.
You don't want to be blasting in so many contracts that you consistently take out the best price and get most of your trade filled at worse levels. If the average offer is 50, say, then if you buy 50 at the market, half the time you will get filled at the offer and half the time some of your order will get filled at a tick higher than the offer. If that is fine with you, then go ahead. If you want to be more sure of not getting a worse fill, then trade in units smaller than 50.
Another alternative is to use a limit order. You won't necessarily get filled, but you won't have to worry about slippage when you do get filled. You have to weigh the opportunity cost of occasionally not catching a trade versus the slippage cost of market orders.
If you do have the good fortune to outgrow the NQ's then add other markets -- the ND, QQQ, ES, SP, DAX, Euro Stoxx, Nikkei, currencies, interest rates, individual equities, etc. There is plenty of liquidity out there.