Countries with a very plural congress/parliment
As Brazil heads toward it's runoff, one striking feature is that it's very multiparty. No party has more than 13/81 and 77/513 senate and house seats, respectively (about 15%). Are there any countries that are similar to Brazil? Are there any that are even more plural?
There are several metrics of how multi-party a congress is. An obvious choice is the number of parties. But this metric does not care about size: A single dominant party plus lots of single-seat parties would score highly.
A more robust choice is how few seats (proportionately) the most popular party has. However, this metric still neglects the distribution between the smaller parties. For example, a two-party dominant 300:250:10 house scores the same as a 300:150:110 house even though the latter has an important third party.
Fortunately there is a metric (one of several) that doesn't have such "blind spots". It is a the HHI index, a mathematical measure of concentration which "combines" the number of parties with how evenly distributed they are. It does not neglect smaller parties. Note: the linked formula rounds the numbers to the nearest percent, but there is no reason to.
Another metric that tries to "cover all the bases" is the Gini index with "wealth" measured in seats. This metric can only compare two countries if they have the same number of parties. To make it work we can add zero-seat parties to countries with fewer than the maximum party count.
Defining a party
Is the Brazil of Hope one party or three? A party should have a fairly uniform platform which is meaningfully different from the other parties. Brazil of Hope contains the center-left green-party and the far-left communist party as well as the standard-left workers party. These are different enough on a left-right scale that in my opinion they are three separate parties.
On the flip-side, US Southern democrats were traditionally fairly right wing and could be considered a separate "party" from the other democrats. The more conservative members have since moved to the republican party.
Of course, left vs right is the first "principle component" of political platforms but there are other "axes" as well. There are also multiple left vs right axes (social, economic, etc).
Defining a congress
Congress or parliament must have a meaningful amount of legislative power. This roughly means at least "partly free" under freedom house.
Given these caveats, we can still make a best-guess as to what a "party" is and calculate the HHI index of a given country (averaging the house and senate for bicameral systems). Are there other countries similar to Brazil? Are there any that are substantially higher?