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"Will 3/16” make a usable sword of a gladius length when I do get to the point of making one?"
It depends on what "usable" is. If you mean slashing at weeds and small branches, Yes. If you mean battling the Persians, No.
Traditionally, a gladius is around .30" at the ricasso. It tapers to the point. Many commercial gladii available online are only 3/16" thick. I would probably forge from 3/8" X1" X12"stock, or grind from 1/4" X2"X24" stock.
exact dimensions and thickness isn't as critical with a gladius, as they are often a heavy sword for their length, weighing 1.5 to 2.5 pounds.. The capulus can be pretty meaty and heavy all by itself. One plus for making your first sword as a gladius is that it can be pretty plain and look good. If you want fullers and decorated capulus, that is fine, but a simple tapered blade and a leather wrapped wooden handle are also fine.
"How many knives should I aim to make before I take on larger projects?"
There is no exact number of knives needed to make a sword. We had a fellow here make his first blade as a wakizashi years ago, and it was fantastic .. but he was a NASA engineer.
Most folks make knives a good year before tackling a sword. The biggest thing to remember is a sword is not a long knife. Swords have very different symmetry and balance. I would venture to guess that the majority of knifemakers never make a sword. That said, it is really cool to make a sword and show it to friends, saying, "I made this."
