UPDATE - July, 09 2024:

This is now being set in your solr.xml file that resides in your solr home directory (where your cores / collections are):

<solr>
 <int name="maxBooleanClauses">90589</int>
 <shardHandlerFactory name="shardHandlerFactory"
 class="HttpShardHandlerFactory">
 <int name="socketTimeout">${socketTimeout:600000}</int>
 <int name="connTimeout">${connTimeout:60000}</int>
 </shardHandlerFactory>
</solr>

If you get the Solr error: "too many boolean clauses", please try to check your synonyms.txt, stopwords.txt or protwords.txt, and try to make those files smaller.

In other words, Solr is trying to apply boolean clauses for each one of those words that are found in any of those files, depending on your schema.xml configuration.

A quick fix, is to remove some of the synonyms from synonyms.txt or other words from the other txt files shown here, and/or you can also take a look at your schema.xml and make sure that your synonyms, stopwords and protwords are configured properly in the chain of tokenizers and filters for your fieldType definitions.

Also, try not to apply synonyms.txt at indexing time, as that replaces many of the original words with their synonyms, considerably increasing the size of your index, and also make search far less relevant in some cases.

Here's an example setup for the synonyms.txt usage, in a text_general field, that we use for our Web Crawler Site Search solution:

<fieldType name="text_general" class="solr.TextField" positionIncrementGap="100">
 <analyzer type="index">
 <charFilter class="solr.HTMLStripCharFilterFactory"/>
 <tokenizer class="solr.WhitespaceTokenizerFactory"/>
 <filter class="solr.StopFilterFactory" ignoreCase="true" words="stopwords.txt"/>
 <filter class="solr.WordDelimiterGraphFilterFactory" catenateNumbers="1" generateNumberParts="1" protected="protwords.txt" splitOnCaseChange="0" generateWordParts="1" preserveOriginal="1" catenateAll="0" catenateWords="1"/>
 <filter class="solr.LengthFilterFactory" min="2" max="100"/>
 <filter class="solr.LowerCaseFilterFactory"/>
 <filter class="solr.SnowballPorterFilterFactory" protected="protwords.txt" language="English"/>
 <filter class="solr.RemoveDuplicatesTokenFilterFactory"/>
 </analyzer>
 <analyzer type="query">
 <charFilter class="solr.HTMLStripCharFilterFactory"/>
 <tokenizer class="solr.WhitespaceTokenizerFactory"/>
 <filter class="solr.SynonymGraphFilterFactory" ignoreCase="true" synonyms="synonyms.txt" expand="true"/>
 <filter class="solr.StopFilterFactory" ignoreCase="true" words="stopwords.txt"/>
 <filter class="solr.WordDelimiterGraphFilterFactory" catenateNumbers="0" generateNumberParts="1" protected="protwords.txt" splitOnCaseChange="0" generateWordParts="1" preserveOriginal="1" catenateAll="0" catenateWords="0"/>
 <filter class="solr.LengthFilterFactory" min="2" max="100"/>
 <filter class="solr.LowerCaseFilterFactory"/>
 <filter class="solr.SnowballPorterFilterFactory" protected="protwords.txt" language="English"/>
 <filter class="solr.RemoveDuplicatesTokenFilterFactory"/>
 </analyzer>
</fieldType>

You can learn more here.