[Cashtab] Replace magic number with constant

Summary: Replace hardcoded constant of 10 with a value set in constant config.

Test Plan:
Confirm we are just replacing a magic number with a constant
npm test

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D15049