I believe there is a restriction of two spell lists per level (RM2), but no maximum to the total number of spell lists to learn. Purchase 20 ranks in the first spell list for guaranteed success, then up to 19 ranks in a 2nd list each level, but that method makes for worse than a glass cannon... maybe a wet-paper cannon?
I'm not even sure about that - as far as I remember, the only restriction was that you had to purchase 20 ranks in a list before being able to buy ranks in a second (because the stat bonus applied to the list acquisition roll), but I don't remember there being an upper limit of 2 per level (in any case, you needed to have at least 41 DPs to buy a rank in the third one, which was possible but required very good stats to all five DP stats.
The provision to buy more than one list is in RM2 ChL&CL p50, and I don't see any explicit limit. The rule is provided for characters who want to develop a second spell list, but there is no "it is not possible to buy more than two lists in a single level" or similar sentence.
The example that follows states that "20 points is usually 2/3 or more of the total development points available to a character for a given level", which explains why the rule was not extended - except having more than 40 DPs *is* technically possible.