Basically you need to look at your winrate and your confidence.
- If you are a losing or break even player, stick to one table. Any mistakes you are making will only be compounded by multi-tabling.
- If you're not confident in your game and trying to learn, stick to one table, same reasoning as above.
- If you are a confident winning player at a given limit, try adding a table and see how it affects your winrate. Start off with 2 tables - don't expect your winrate (in terms of BB/hr) to double, but it should be higher than your winrate for 1 table if you're to continue adding tables. For example, say you're winning 2BB/100 on one table, but only 1.5BB/100 when 2-tabling (because of missing out on the occasional read, rushing the odd decision, etc). Your BB/100 has gone down, but as you're playing twice as many hands your winrate has actually gone up.
- If you find your winrate is actually dropping, switch back to 1 table or move down in limits if you're insistent on getting multitabling practice.
- Add another table if and when you're more confident playing the number of tables you're at, and repeat the analysis above.