Hi,
you can find the resources here: tpv-shared-database/tools.yml at main · galaxyproject/tpv-shared-database · GitHub which is shared across Galaxy instances and then local overwrites for EU here: infrastructure-playbook/files/galaxy/tpv/tools.yml at master · usegalaxy-eu/infrastructure-playbook · GitHub
${GALAXY_SLOTS:-1} is a bash syntax and just states that if the environment variable GALAXY_SLOTS is not set, then default to 1 core. We do set GALAXY_SLOTS with the values given in the linked resources files.
What is your scientific question you want to answer? There might be a better way to solve your problem that does not involve mafft.
Ciao,
Bjoern